tube preamp in SS receiver loop

If you go with the buffer don't forget to expirement with using it plugged straight into the main ins on your Marantz - even if you bought it with the full intent of using it as you describe above. The difference in sound quality just may surprise you...or not, but it would be worth it just try it out long enough to see for yourself. But, I believe you're on the right track and good luck with it all, either way.

Thanks. That was my intention, to plug it into the receiver's main ins, with the preamp outs feeding into it. But it will be fun to try different configurations. Unless it sounds awful. In which case it won't be all THAT much fun.

Other than using a tubed based source phono pre amp or CD player you have a beter chance of problems than improvement. I am not saying you will fail but the source will add great tube sound with almost no chance of failure. You might find a dealer or friend whith something to try or if you buy something and don't like it you can sell it on Audiogon. I build tube gear and upgrade anything, Improve the power supply is all I would do to a reciever. That improves anything.

This is very simple. Your Marantz has a mediocre preamp--at best. Furthermore, the tape ins and outs are buffered and will swamp the advantage of an additional tube buffer to a significant degree. They guy that said keep it simple is correct. Buy a decent used tube preamp. Plug your sources into it. Hook its outputs to the main in on the receiver. You're done. One volume control and less gain stages than all the other schemes.
By the way, a tube component that's "warm" is either defective, needs to be retubed or you're comparing it to a real piece of garbage--be it solid state or tube (either architecture can sound like crap no matter what anyone tells you). Tubes are revered for their transparency, liquidity, detail and so on. The idea that they are warmer is a misconception. They're either natural or they're colored ( or veiled, or muddy and on and on).
You have two problems: one is $. The other is you must listen to the actual unit youre buying. I've repped dozens of high-end lines--still do. I saw a mint Golden Tube SE40 rep sample that had almost no time it it. Unfortunately, it had no life to the sound--dull and lifeless. Real music is dynamic and exciting. The unit sat in the rep's closet unused at least 15 years. It probably needed re-capping since tubes hold up well if unused.
Call Andy at Saturday Audio in Chicago and Holm Audio in Woodridge, IL. Both have used tube preamps. Try to get return privileges. Tell 'em Frank Malitz sent you and tell them what you're trying to accomplish. By the way, any good separate preamp--solid state or tube--will outperform your receiver's preamp section. When I was at Onkyo, we could only make decent preamps--never really good ones. We simply didn't know how; a good phono stage is very difficult to design. The line stage is easier and the power amp, easiest if all. I'm generalizing but for mundane Japanese brands (which I also rep), high performance separates are not their strength.
Best of luck.

Vesuvio,
You are on the right track and I agree with Fmalitz. I have direct experience with what you want to do. When I wanted to do separates years ago I could not afford a tube pre and power amp. Plus I have a fairly large room and I like orchestra and organ recordings, played at a decent level. That cut out the tube power amp, unless I could afford a Wotan or other model for 20K!

I had a Pioneer surround receiver and I was ready to spend money on a tube pre. I bought an Audible Illusions Model 3 (I have since upgraded it to the 3A), plugged it into the main on the receiver and used the pre for switching inputs and volume, etc. I remember bringing my receiver to the High End store and switching between the receiver volume section and the AI Pre. I could not believe the difference, it was amazing. There are a lot of used tube Pre's out there. Also new ones too.