IMHO...it all depends on the speakers you're running, amp design, etc. In my particular case, when comparing EL34-based monoblocks to 6550-based stereo amp by the same manufacturer, using very similar design...I preferred the 6550-based amp.
It didn't come down so much to performance at the frequency extremes - it was more that the EL34's midrange occasionally sounded grainy compared to the 6550, and that dynamics seemed compressed with the EL34's. This is all partly due to the fact that both amps use 4 power tubes per channel, so the 6550 just has more power (and my speakers are able to do something with the extra power) - but it also comes down to resolution IMHO. The 6550's just seem to be higher resolving to me.
It didn't come down so much to performance at the frequency extremes - it was more that the EL34's midrange occasionally sounded grainy compared to the 6550, and that dynamics seemed compressed with the EL34's. This is all partly due to the fact that both amps use 4 power tubes per channel, so the 6550 just has more power (and my speakers are able to do something with the extra power) - but it also comes down to resolution IMHO. The 6550's just seem to be higher resolving to me.