I just bought a Audio Research D-90.

Usually a tube amp puts out the same wattage into any load, unlike a SS amp which doubles down until it runs out of current, often 4 ohms and in inexpensive amps somewhat above that. A few big tube amps even have 2 ohm taps though but their power is still the same, just more current.

Using 6L6's and 5881's will typically produce about half the power of a 6550. I've never owned your ARC amp - I was unaware that it would take 6l6/5881's, which along with EL 34 have a similar wattage rating, as do E34L's and 6L6GC's (about 5 wts more), all things not being equal. Don't assume they do, check with ARC, if you don't know for sure, and they will certainly require different bias points if they do. And that will vary as well.

Hope that helps a bit.

Thank you for you help. A.R. made 3 model's of D-90's. They are D-90 (4 pairs of 6550's),D-90-revised (4 pairs of 5581's and the D-90B witch also takes (4 pairs of 5881's.
I'm new to tubes,but I do have a C.J.45mv-a1 that I bought a few months ago. I've been rolling tube and driving tube sellers on ebay nuts with ?'s Is there a book out there what would explain the differant qualities of the tubes?
My Mac pre-amp has 2 sets of outputs, So I'm thinking of bi-amping with the C.J. for mids and highs and the A.R. for the lows.I've got several active x-overs so I thought what the he--.
Any input would be more then welcome. Thank you for your time and trouble,Bill

Very minor correction: while the D-90 put out 90 watts per channel into any load, the D-90B is listed by ARC as 80 watts per channel.