if i could have only one amp, it would be my mesa baron. because it's several amps in one!

you can run it full triode at 55wpc. or 1/3 triode-2/3 pentode at ~85wpc. or 2/3 pentode-1/3 triode at ~120wpc. or full pentode at 150wpc. and, you can select negative feedback at 0db, -2db, -4db, or -8db, at any of the triode/pentode settings. you can also swap out power tubes, using 5881's, 6550's, kt88's, el34's, etc. (there's a specific mod for this, but according to the folks at mesa, you can also do it w/an unmodded amp, but you have to be more careful w/the bias settings.)
this is a great sounding amp that will work w/almost any speaker. it has the sonic beauty of tubes, but will also drive difficult-to-drive speakers that might normally want a solid state amp. no longer made, expect to pay ~$2k for one in good shape on the used market. mesa boggie will still support mods & repairs on it as well... i would look for one that has a slotted chassis - these later iterations offer better cooling. either way, slotted chassis or solid chassis, they are built like tanks, & wery reliable...
http://mesaboogie.com/Product_Info/Out_of%20_Production/The_Baron/the_baron.htm
doug s.