I think you did a really good job in figuring all this out for yourself. Trust the results you came up with. Trading the 6300 in for a preamp seems like a pretty safe bet. It makes sense.
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I agree that the MA6300 isn't needed any longer.
The C2500 is a tube preamp with MM/MC phono inputs and a DAC section - that is the enhancement over the C2300. C2300 sounds wonderful (to my ears anyway) and was very well reviewed.
C48 and C50 both have MM/MC phono and DACs; C50 has more EQ bands and gets meters. Not sure if there are enhancements to the DAC section in the C50 over the C48 - haven't checked the specs. My C50 is one of the best purchases I've ever made.
Note the pass-thru feature could allow you to send variable output from the D100 directly through one of the preamps. That could offer you more flexibility than you currently have.
All units are in current production, so you can easily compare specifications on the McIntosh Website.
When you were using the variable outputs of the D100, what settings of its volume control did you typically use?
If you were using relatively low settings (i.e., large amounts of attenuation), and if (as seems likely) its volume control function is implemented in the digital domain, you may find that the sonic character of the variable outputs would be much better if you were able to use settings closer to the top of its range. A way of accomplishing that would be to put a pair of Rothwell attenuators at the inputs of the MC275. You would select either the 10 db or 20 db version of the attenuators depending on how far down you presently have to set the volume control when using the D100's variable outputs.
Considering the modest cost, that seems to me to be an experiment that is worth considering, before investing in a new preamp. Especially if you presently have to set the D100's volume control in the lower part of its range.
Some people have reported adverse effects on dynamics when using the Rothwells, but many others (including me) have used them with no perceivable side-effects.