I am very grateful for all of your thoughtful responses, and am learning a lot! I have been away from all this for a few weeks due to a frantic new job with ungodly deadlines, and before reading some of the more recent posts here -- in fact not realizing they WERE here -- I acted on the Yaqin ms12b tube phono stage and line preamp, which arrived the other day. If it doesn't work for me, it can go on sale. I would like to have the money to do better than trying to cobble it together with my modest vintage SS receiver, but maybe that will come with time.
I've just begun to experiment (a wonderful way of not worrying about work!). What I have discovered: If I plug the Yaqin directly into my receiver's PRE-OUT MAIN-IN loop, all the receiver's tone and volume controls are disabled for the phono, which of course goes directly from the RIAA input on the Yaqin preamp to the solid state MAIN IN. Tone and volume do work for other sources, I guess because they go through the SS preamp before going into the Yaqin, and I do have the Yaqin volume control.
If I plug the receiver's TAPE OUT into the Yaqin LINE IN and the Yaqin output into the receiver's MAIN IN, resulting in a TAPE OUT-MAIN IN loop, the receiver's tone controls are bypassed for all sources and the receiver seems to act as a source selector without modifying the signal, so that might be the purest solution. But I may still need tone controls at this point.
So far, with the Yaqin going directly into the power amp--without the use of tone controls and the ability to temper the treble--I find it harsh and tinny, much more so than the SS receiver on its own--which I was trying to mellow and soften. (I am not yet a purist!) Of course, I still have new stock Chinese tubes in it, which are not supposed to be all that great.
On the other hand, if I put the Yaqin into the Marantz TAPE-IN TAP-OUT loop, where I have the advantage of the receiver's tone and volume controls, the sound is more pleasing--still not sure how much better it is than the plain ol' Marantz is, but maybe tube rolling, which I have never tried, might be the next step.
This is all quite confusing to me, and one day I may bite the bullet and go for an integrated tube amp. But in the meantime, what about just leaving the tube pre-amp in the tape loop, with access to the receiver's tone and volume controls? At least for now? Followed with better tubes fro the Yaqin.
Or can anyone think of a better configuration? Thanks, all, for sharing your knowledge and expertise.