I spent some time creating guitar master, vocal master , harmony master and drum master.  
 
I have overheads and individual drums(drums also sending to compressed drum sub called compressed drums for parallel processing) going to drum master.
 
its nice to control overall drum volume with one gain, but it seems to degrade the quality of the resulting mix in the master bus.  
 
Anyone else know or experience what I am saying?  when I set each drum track to output to master bus instead of going thru an "eq summing bus" or master drum bus it sounds much better.  
 
IF no one else has experienced this than it may be some issue with my headphone/output setup or the RME mix. Anyway the ability to do sum volumes for an entire instrument type is not worth the audio degradation in some cases.  (I don't have anything on (like the pro channel) on these summing busses, for the most part they are just summing. 

There should be absolutely no degradation.
 
This is assuming you've paid proper attention to your gain staging throughout the chain and there's no processing on your busses which could colour the signal

I agree with Bristol. Using sub buses should have no impact on the quality of the sound.  

All the individual tracks being sent to the master are probably just LOUDER when sent to Master (due to how you have things set up).
 
The human ear percieves LOUDER sounds as BETTER.
 
To test this (and disable all effects on the Master Bus and your sub bus for a proper comparison... including Prochannal and FX Bin) route all your sub bus tracks to the Master and note the output level on the meters. Then route them to your sub master (which should be routed to your master) and make sure the meters are gauging the same output levels.
 
They should sound the same.
 
I am talking about using the main output of the tracks. Not sends. A bunch of other stuff needs to be watched when using Sends like pre/post status, send levels and whatever.
 
The Master bus is just a bus exactly like your sub bus. Without any effects absolutely NOTHING will be lost or added between your sub bus and master in a digital environment if the levels are the same.
 
Essentially... your ears are likely playing tricks on you... or you are not using the same levels... or you have effects in there screwing things up.

Or, you have tracks going to more than one sub bus. Also, you need to take care of what your compressor is doing, in case it is causing phase issues.
 
Submix busses never alter the sound by themselves.
 
Regards,
Dan
 
 

+1 on possible phase issues.

kzmaier
+1 on possible phase issues.

+2, especially if the drum source is AD2 rather than recorded drum tracks or some other synth. There is a known issue with sends from AD2 tracks being out of phase with the outputs by 64 samples due to a PDC problem.

Technically it makes no difference, so if you're hearing a difference that isn't imaginary, it's because something else changed than just the summing part.
 
This is really all that happens with the summing part:
 
1+2+3+4 = (1+2)+(3+4)
 
Once you start adding other processing, you can obviously get different results, but that has nothing to do with the summing to sub busses vs. everything going to master.

Thanks so much , it looks like I had at least two issues.  I was interpreting louder and better, and I was using a send from the AD2 main output track to a compressed sub.  The guitar sub sounded fine I think (when i upded the fader).
I am now just compressing the live drums and I rendered a part of the AD2 output to track, put it next to the live drum audio tracks and compared in a deep zoom.   I used drum replacer to create the midi for AD2 and by rendering it and putting next to the real snare I found I had to slide the midi up/back in time exactly 10 ticks to be right on with the live snare.  It worked beautifully.  Anyway, I don't think the subs were the issue, thank you for confirmation.