Unless that Kenwood has been checked out on a bench, it probably needs some capacitors replaced, there's an excellent chance it's not performing up to its original specifications, however modest they were. Frankly, I've heard that talk about old stereo receivers being better, and I was around when the old stuff was new stuff, and in every case I can think of I'd rather have a new power amp rather than a receiver. IMO there's just no comparison.
I'm not surprised that a tube amp might make the RSBs sound better. Infinitys were known as having a rather bright high-end, especially the RSBs, and a tube amp might cause a nice system frequency response roll-off up there. Of course, it would depend on the tube amp and the speaker. I think that's part of what attracts audiophiles to tube amps, the fact that the amp-speaker interaction is much more unpredictable than with solid state amps, so you don't really know what you're going to get in terms of frequency response until you try it.
Frankly, for about five hundred dollars in carefully chosen used equipment I think you blow the doors off of the Kenwood-Infinity combination. For $1000 you could get some really good sound.