I loaded my Twin Reverb up with 5881s (plus a few NOS preamp tubes) and the amp really opened up. the 6L6s are great, but the 5881s just have character...maybe it's the way I have them biased as well...
All though the EL34's give a nicer natural overdrive, I prefer 6L6's because the clean tone is more sparkly and jangly and they give you an excuse to buy a nice overdrive pedal.