I did a quick comparison between using a tube preamp (bypassing the internal preamp) into a MOTU 896mk3 Hybrid and going direct into the MOTU and using the built in mic preamp. 
 
The "quick and dirty" test, the MOTU preamps win. 
 
I recorded my wife singing, first into the tube preamp and then direct and compared the two on playback.
 
I used an ART PRO MPA II preamp, with High Plate Voltage for the 12AX7 tube.  An AT2035 mic.
 
With the ART preamp going into a channel on the MOTU 1/4" TRS input (bypasses the internal preamp) and the voice seemed to have a trebly edge to it.  Going direct into the XLR input on the MOTU (using the built in preamp) the recorded voice sounded exactly like her actual voice and didn't have the trebly edge.
 
This was a very unscientific test.  Mainly I wanted to compare the two with my new MOTU. 
 
A side note, I've seen a mod for the ART PRO MPA II, that changes an op amp and I'm not sure what else.  Considering my comparison test I'm going to pursue the mod and see what happens.
 

I have the same preamp and I love it for anything acoustic. I would almost rather have some sheen to trim off than  going in with less top end but that's me. I havn't done the mod either, however my model is more recent and I think ART has dome some of their own improvements on more recent models. 

I have the ART MPA II and a MOTU Ultralite MK3 Hybrid (which has 2 preamps).
 
there are differences between the two preamps, absolutely - I would be surprised if someone said there weren't (or rather I'd think they didn't know what they were doing ).
 
but I would not say that the MOTU preamp is "better" - only different.  the MPA is designed to add tube distortion to the input, so if you're trying to compare which one sounds "better" it might be that the MOTU preamp sounds "cleaner" and that would definitely be correct.
 
I use the MPA for most things, but if I want a clean sound I'll use either my True Systems P-Solo or one of the MOTU preamps.
 
doing the Burr Brown op amp mod to the ART MPA will not change the color between a clean preamp and a tube/starved plate preamp.  It will change the sound of the MPA, but it will still not sound as clean as the MOTU's preamp because it's not supposed to.
 
I suspect you may be disappointed with the Burr Brown mod.  If you want true clean preamp, the MOTU isn't bad, but if you want more, get a True Systems P-solo or a Grace m101.  that would be money better spent if you don't like the colored sound of the MPA and prefer the cleaner sound of the MOTU.

Preamp flavors are aesthetic... so YMMV
 
I have a GAP Pre-73 (Neve clone) that IMO sounds better than the UA-610 (classic tube design).
Transient response is significantly better, it's significantly quieter, etc.
Had the UA-610 for a long time... then finally sold it.  Was never getting used...
 
IMO, There's only a small handful of inexpensive mic preamps worth buying.
 
 

In ref to the mod(s) for the PRO MPA II.  The Burr Brown (TI) OPA2134 chips made a big difference modding Peavey Nashville 1000 and Nashville 112 amps.  Replaced all the chips on the preamp board with the OPA2134's and it cleaned up the amps and removed the Peavey "mid honk" that a lot of their amps have.  I've also seen a lot of comments about replacing the cheap factory Chinese 12AX7's with better tubes, such as NOS RCA 12AX7's and even the current Tung-Sol 12AX7's, that seem to make a big difference.
 
As far as the Plate voltage, I was under the impression with the High Voltage option it was not operating in a "starved plate" mode?
 
I have a recording session scheduled tomorrow, with a male singer.  I'm going to test it again with the male voice and see how much difference there is. 
 
I don't use the preamp for instruments.  I have Sans Amps that I use for bass and pedal steel and a Roland 80GX amp that I use for my Tele.  I don't use any distortion - I record "traditional" country music.

fireberd
In ref to the mod(s) for the PRO MPA II.  The Burr Brown (TI) OPA2134 chips made a big difference modding Peavey Nashville 1000 and Nashville 112 amps.  Replaced all the chips on the preamp board with the OPA2134's and it cleaned up the amps and removed the Peavey "mid honk" that a lot of their amps have.  I've also seen a lot of comments about replacing the cheap factory Chinese 12AX7's with better tubes, such as NOS RCA 12AX7's and even the current Tung-Sol 12AX7's, that seem to make a big difference.

yes, I agree with that.  from your initial post, however, you're comparing a clean preamp to a tube preamp which, IMO, is not a fair comparison since they're completely different beasts which are designed to sound different.

 
As far as the Plate voltage, I was under the impression with the High Voltage option it was not operating in a "starved plate" mode?
 
 

yes, in high voltage mode it is not a starved plate - the "/" in my comment was meant to include both starved plate and full voltage tube preamp designs as a comparison to clean FET designs.  I did not mean to infer that the high voltage option was starved plate.

Actually it was a fair comparison.  I just wanted to see what the difference was between using an external tube preamp and the built in preamps.  And, as I initially stated it was very unscientific.  I used what I have.  I just have a home recording "studio", not a full blown studio (which in that case I would be using all high end preamps and equipment).
 
Before getting the MOTU, I was using a Roland Octa-Capture and the tube preamp seemed to be a plus on vocals.

After doing a recording session with a male vocal, yesterday, direct into the MOTU I've decided the ART PRO MPA II is surplus and will be sold.  I still have a Presonus Tube preamp (not a starved plate tube type) and if I want or need a tube preamp I can use that.
 
 

Did you use the TRS inputs on the rear of the MOTU.  They are the only inputs that bypass the internal Mic pre amp, not the front panel.  I guess you did.
 
What concerns me about the ART pre amp is how cheap it is for a two rack space unit like that.  Makes me wonder about the quality of things such as the power supply (very important!) the choice of transformers etc..
Choice of components in general.  A decent (mono) valve pre could cost ten times that amount!
 
According to the Sound on Sound review of the ART in order to get the VU meters working normally or showing a reasonable deflection the unit had to go into quite a heavy drive mode.  That may be generating extra harmonics hence you may be hearing that as extra top end.  They said they had to back it off quite a bit in order to get a smoother more rounded sound.  You may not be using the ART correctly perhaps.
 
http://www.soundonsound.com/reviews/art-pro-mpa2
 
I don't disagree with you though.  I have got a Yamaha digital mixer too with real nice Mix Pre's and they actually sound very transparent and do not add anything of their own which is sort of the way I like it.  In most cases when I patch a quality microphone to them direct I feel I am hearing a very real sound.
 
Any external valve preamp is actually adding distortion to the signal and there is no other way to look at it.  People think they have to record through a valve pre but in fact that is not the case. If you do you are imposing that sound over everything you track and it could be said that in some cases it is not the right sound at all.

Yes, I used the TRS inputs.  I'm not driving the ART very hard as the VU meters hardly move (one thing I noticed from the time I got it).  I was recording at 24 bit/44.1Khz, -12, in Sonar for the tests.
 
As I noted, I didn't notice the "tube edge" on the Roland Octa-Capture and playing with the Impedance control on the ART sort of "tuned" the vocals. 
 
In its place the ART can be a plus, like it was for me with the Octa-Capture.