I have a Cary Rocket 88 for sale right now, and I would absolutely NOT recommend adding tubes to a HT system.
Why? For one tubes go bad after a certain amount of use and that use in amps is about 1/10th the time tubes tend to last in tube preamps.
But aside from this tube question, I have done a lot of experiementation in the past with different amplification driving rear channels. My conclusion is that it really does not matter A LOT as to what quality of amp one uses to drive rear channels. Rear channels are not recorded well at all (compared to say 2 channel recordings). A lot of the stuff that goes on in the rears is echo and background noise. I could safely say that I could not tell the difference between a 100wpc Sony amp driving my rears (a few years ago) and Monarchy SE100 deluxe monoblocks driving my rears.
Now if you had a precise HT setup in a large room... well that may be a different story. You might need serious power driving the rears (and I would not choose the Rocket 88, it is a darn fine 2 channel tube amp, but at 40wpc max it does not drive 90db speakers terribly loud well).
I run 7.0 channel sound now with a receiver driving all the channels except the L & R front channel (run through the two channel system). The sound is very good. About the only thing I would upgrade is center channel amplification. Center amplification does matter a lot, and this is an area that my system is lacking.
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OOPSE I just discovered I misread your post... You want to add a tube amp for your L/R channels. I would not recommend it due to the tubes burning out issue. I would go with something a bit more powerful and solid state. Lots of SS options out there, depnds on how much you want to spend.
KF