Tube amp / SS pre or SS amp / Tube pre?

If you want to mix and want the tube sound you should drive your speakers with tubes and that's where they're most heard.
You should also pick the appropriate SS pre- preferably MOSFET or JFET that indentically to the tubes amplify voltage instead of current.
In tube-pre case you should ask poweramp's manufacturer if it's OK with tube preamp i.e. there should be an input impedance sufficient enough preferably >40k. Many Pass amps are not successfull with most of the tube preamps due to the very low input impedance.
You can also try passive/tube combo if you've ever thought about it and that's my favourite.

Somone who's opinion I respect, told of an unlikely but suprisingly good system with Mark Levinson pre going into VTL mono amps.

Gee Unsound, that describes my system as of last week...
(Well, maybe not the "unlikely but surprisingly" part - and yes, it works fine :-)

I have previously posted the same thing Marakanetz is saying: despite the popularity of the tube preamp/SS amp pairing, I tend to think it's the power amps where placing tubes (power tubes) will give you the most of what tubes do that is distinct from transistors. But considering how fine your current SS system must be (and not understanding your qualifier about where the analog front end winds up) - and considering that what you're talking about is a second system, not a replacement - why not go whole hog if you're curious, and do a totally tubular new rig?

Marakanetz and Zaikesman suggestion of putting solid state pre before tube amps usually offer more opportunties for appropriate impedance matching.

How about a CD player with a tubed output stage? I'm not a fan of SS amp w/tube preamps, too much of a compromise and the worst of each setup is often emphasized, at least in my experience. A good tube CD player can you the magic of each without making so many tradeoffs.