I also have 2.5's driven with Conrad Johnson amps and the sound is equally magical for me. I don't have much experience with the new Response series, but if its like the older line (2.5 vs 1.5) then the differences will be a dramatically improved bottom end (deeper, with a nice mid bass richness) and a more realistic and fuller mirange. Tonally, the 1.5 and 2.5's are similar, but the overall depth of the sound is substantially different. It would be worth finding a dealer that sells the D25 and see if the same difference exists between it and the D15's.
I've had the 2.5's for 4 years and have no plans to replace them. I love these speakers :-).