tube preamp in SS receiver loop

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.

I seem to agree with some of the principles here, but not many of the recommendations. Keeping the signal simple is a good one. Ditching the mid-fi receiver is another (sorry).
Aside from a vinyl record player, most input devices, even quite ancient ones e.g. cassette, reel to reel output at "line level" of around 1v and have fairly standard output impedance (a bit brave, but bear with me). Some newer inputs e.g. DVD, BD, DAB may substantially exceed this (often up to 2v). Consider what a pre-amp "does". Given decent inputs, all a pre-amp really does is to switch from one input to another and adjust the volume (downwards) from line level (or whatever the input level is). So all you need is an input switch and a volume control, neither of which involves tubes at all. It is partly audio "philosophy" but IMHO, at best a pre-amp should be passive and should interfere as little as possible with the sound. So, you can get high quality input switching modules from China with gold-plated relay contacts for $25 (e-Bay). You can spend $700 on a Stephens&Billington transformer-based volume control or $500 on a fixed resistor stepped volume control, both of which are excellent solutions, however, on a budget, the best value for money solution I know is a motorised ALPS Blue-velvet volume control from Germany for $35. You can then get a remote control board and transmitter to switch the inputs and control the volume via the motor from your armchair for another $20 (e-Bay). This leaves 2½ issues: Phono stage - get a cheap solid state stand-alone one for $30 e.g. http://www.maplin.co.uk/stereo-phono-pre-amplifier-28732 . You can spend $000's on a phono stage, but we are just experimenting here, right? Second issue, tone control: Tone control should only be used to correct deficiencies in input quality. It is not required for phono, any digital input like CD or FM radio. It can be useful with poor cassette tapes or reel to reel tapes which have deteriorated over time and repeated playback. Again here, what adjusts tone is essentially inductance, capacitance and resistance (LCR) networks - no tubes. These are pretty simple, but if you don't feel comfortable making them, you can probably get a cheap solid state or passive unit. Plug it in between your deficient input and the switching unit. This yanks the budget up a bit, but Musical Fidelity offer a stand-alone tube-based tone control unit called an X-Tone. You can get one of these (the Mark 2 version) for around $200 on e-Bay. Last issue - impedance-matching: All this takes is two resistors per device before the input switch - again no tubes - and I would venture to guess you won't need to do it anyway.
So we have elimiated a lot of noise and distortion (and tubes) from the pre-amp zone. If you still want tubes (I use them), you could get a tube-based power amp. You can get an entry-level built module from China (e-Bay) for $45 or a more professional finished version in a case for $200. You can probably find some worthwhile tweaks and mods on the internet if you want to upgrade this to sound better.

I have a Linn, Majik DSM which is an excellent digital music player, (integrated), with all components connected to use it's volume and switching, (and great DAC), and then fed via pre-out to a MasterSound 300BPSE which is also an excellent, Italian integrated which I am only using the Line-in so it's role is strickly amp.

The sound is pure and magical. No coloring but the staging and separation is the best my aging ears have ever heard.

Higher end than what your testing but an excellent higher mid-range configuration that simply delivers beautiful sound, (and the amp is only 24 watt)!

My speakers are custom built, (based on SEAS Exotics), and at 91db efficiency, let me hear if anything is amiss and so far, the only disappointment has been some the tubes I have purchased, but then that has also allowed me to find a combination that has had me in tears of joy.

Your idea works and delivers beautiful sound.

Cheers

Bob