The ART MPA II is included in the group of preamp graphs that are on my website.  You can see the harmonic spikes of the 992hz test tone and their relative sizes.
 
See article: http://www.tedlandstudio.com/#!the-pain-and-the-gain/zqysw
 
 

Doesn't look much difference in any of the graphs you have posted.  My conclusion is my ART "may" be useful in some applications but not for my home studio use with the MOTU.   I am not running it very hot, actually very low level, and the noise floor was not an issue.  Just clean audio, as I only record traditional country which has no overdriven instruments, the bass is still used for rhythm not a "lead" instrument, the singers still sing -not yell, etc.
 
 
 
   

starved plate = fuzz

I didn't use the starved plate option.  I always used the high voltage option (which according to the manual is "regular" amp).
 
I had a Presonus Dual Blue Tube preamp and it was starved plate and really did nothing.  It was sold on ebay.
 
 
 
 

unless it's class A circuit,
typically all the tubes do, is add harmonic distortion.
 
me, i think i'd rather have one of each... class A, driven to distortion,
or something discrete and clean, so i can change up the sonics.
 
my high end preamp is a A Designs Audio-MP-1,
but i also have a cheapy ART DPS,
and sometimes i prefer it for it's grungy sonics.
 
but what jim said earlier, rings true to me,
there are not many really decent 'cheap' mic preamps,
seems the cut off for this is about $500
 
the RNC RNP comes to mind...
 

I'll agree you have to pay big $$ for "studio grade" or "audiophile" grade preamp.  I don't have a "starved plate" type design anywhere, even for my guitar amps (except for one Carvin amp, all of my guitar amps are solid state, such as the Peavey Nashville 112 steel guitar amp).

I guess I missed this discussion before, including the following from Jim.
 

Jim Roseberry
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.

 
Good to know...especially since I have a GAP Pre-73 MkII:)

I have an Art MPA Gold and I've yet to find a source to which it adds anything positive to the signal. Probably heading for eBay.

After several months of using the MOTU without an external preamp (although I still have a couple) I'm still convinced I don't need a preamp for vocals with the MOTU. 

Jim Roseberry
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.
 
 

I ´m also using the Gap Pre 73 for years now.
It gives a nice colour and tightness used as DI Box for Bass.Also as a
preamp for a more neutral SE 2200  mic it´s very useful in giving some
overtones and (wanted) dirtyness to the signal.The neutral preamps
of the Focusrite Saffire Pro are a good addition for my needs.
Don´t see the need to spend thousands of bucks for a another colouring preamp.
The GAP is imo a very good one.