Lovepianos, Tennis is quite right about the stock tubes (horrendous ;--) But they're not so bad you couldn't enjoy the amp's wonderful attributes. If your result is missing both image (higher freq) and bass, I'm inclined to believe there might be an impedance mis-match between the Rotel and the 275. That said, the 275 has a fairly high input impedance (I think! It doesn't say in the manual) and should accept most preamps.
If you want to try other tubes, see my report on tube rolling w/ the MC275 at:
http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1175279050&openmine&zzNsgarch&4&5#Nsgarch
.
If you want to try other tubes, see my report on tube rolling w/ the MC275 at:
http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1175279050&openmine&zzNsgarch&4&5#Nsgarch
.