You can't universally connect the preamp stage of a DAC into any power amp and expect great results. Sometimes you can but not always. If the amps are too sensitive or you have an impedance mismatch, you may not get a great result.
Tube vs R2R, again, don't assume all R2Rs are warm and engaging. I can show you R2Rs that are bright and cold. The analog stage matters greatly in terms of imaging and soundstage. Some R2Rs have tube output stages.
It is tough to make generalizations. There is a common belief that R2Rs are more musical, have more texture but are generally less detailed than delta-sigma counterparts.
Delta-sigmas have a reputation for being brighter, more clinical and more detailed and tubes will tame the brightness.
These generalizations are not true and it really comes down to implementation. I can show you warm, musical delta-sigmas and cold and bright R2Rs. The Rockna Wavedream Signature is one of the most detailed DACs I have ever heard (if not THE most) despite being R2R. Bricasti is one of the most musical DACs I have ever heard despite being a chip based delta-sigma.
What is your budget? What tube gear do you have? XLR or SE? This will help zero in on some items that you might want to consider.
