Peter, sorry about the delay in posting a reply to your question. My view of the three CD players I have owned with the variable/volume control is:
Capitole is awesome sounding in that it produces beautiful music. This is not to be confused with neutral or 100% accurate music. It lacks the detail of many of the other better caliber players, but is truly beautiful to listen to. It has what in my opinion is the optimal sound of tubes. This is not bloaty or overly softened music, but still a golden sound. Soundstage depth, width and height are very good to excellent, but focus is not as accurate. Bass is good, but not as tight. Transients are good, but again maybe not quite as fast on the leading edge, but very good on the trailing edge. I loved this player, but am somewhat concerned about reliability. The moderately prices used ones can be had for about $3,000 (Mk. 2 or 24/196 versions), but I know that certain parts are no longer available for repairs. The SE versions are a bit more in the $4,000-$5,000 range and newer and hopefully more reliable.
The Wadia unit has a digital volume control, whereas both the others have an analog volume control. I like Wadia CDP's in general, but in my opinion, they perform best with systems that are a bit warmer sounding to begin with. The Wadia's are much more detailed than either the Capitole or the Resolution Audio. This is good in many systems, but in my system, the detail of the Wadia did not produce the correct synergy with my equipment (Krell amplification and Wilson speakers). I now have a tubed preamp, so perhaps it would be better in my system now. I like Wadia and have a lot of respect for their players. In the right system, this may be one of the best choices of the three.
The Resolution Audio Opus 21 is a fine player to start with, not in the same league as either the Wadia or the Capitole (or for that matter a Esoteric, Ayre or some others), but with the GNSC mods, this really takes this CDP to a whole other level. I have not changed my power umbilical cord due to my conversations with Steve at GNSC who did not feel that their was a noticeable improvement in doing this (based on a January 08 conversation with him). Steve and I were both a bit concerned that the Opus 21 may be too detailed in my system (again Krell/Krell Pre/Wilson). I did not find this to be the case with a Stealth balanced interconnect to my preamp. Steve feels that the Opus 21 is not as detailed as the Wadia (which he also sells and modifies) and would be better in my system than a new modified Wadia player (at least within my price range).
I am presently using the Opus 21 and it works great for me, even better I think with the Calypso preamp than the Krell preamp. It offers plenty of detail but without getting to point of being etchy/grainy/etc. . . It took me about a month to break it in and it definately improved during this period. While I cannot say this is the best player of the bunch, it has worked out better in my system than the other two. It does not have that "beautiful" sound of the Capitole or quite the hard edged detail of the Wadia, but has really turned out to be a good balance for me. I have not listened to it much with regards to the direct to amplifier option as I have concluded that in most cases this does not work for me (within my system) and I cannot say with this piece whether its performance is better one way or the other. I felt the capitole was better direct and I felt the Wadia was better through a preamp.
I think the Opus is the best deal in terms of pricing for my needs and desires. I think the Capitole and the Wadia are both excellent players, but both have a more personalized sound (warm vs. finely detailed respectively in comparison of the two).
I think the Opus 21 will match a greater variety of systems than either of the other two, which may be nice if you change your components frequently. The Opus 21 will live more happily as your changes take place than either of the other two.
If you buy a used Opus 21 with plans to have GNSC modify it, you will want to speak with Steve in advance. I know he is running out of certain parts of the modifications that are not longer being manufactured. You of course can also buy a new unit from him with the mods (this is what I did) and get a full warranty. He commented to me that at some point in the not to distant future he would have to save his remaining parts for mods of units only that he sells new - to protect and reward his direct customers. I don't blame him for doing this. You should also discuss the differences between the three of these players with Steve. He is a current Wadia dealer and modifier, a current Resolution Audio dealer and modifier and used to be the official service center in North America for Audio Aero. So if you describe your system and goals, he will also be very helpful in making a recommendation.