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.

I think the question is a little too simplistic. While the general feedback provided is probably correct, there are many other factors to consider.

Which tube amp would you consider? Alot of tube amps are designed primarily for high efficiency speakers. If you want something to potentially improve on your Levinsons, you may need to change speakers to expand the pool of canidates further than the big ARCs, VTLs, Atma-Spheres, etc.

With a tube preamp, you can get a taste of tubes without worrying about your speakers. As others mentioned, you do need to check compatibility with the amp, but this may not limit you as much.

From a practical standpoint, tubes are more of a hassle then SS (recurring cost, reliability, noise, heat, availability, etc). However, tubes in the preamp seem to have less of these issues, so this may also tilt things for you in favor of trying a tube preamp.

Frankly, since your happy with your SS system now, I wouldn't recommend changing to tubes. There is no guarantee you will be happier just because you now have tubes. I respect what tubes bring to the table and some people's preferences, but "tubes don't always rule." I've always thought that if you can get happy with a state of-the-art SS setup, then why bother changing to tubes. You may notice some benefits, but I bet you also find some tradeoffs too.

There is not a prevailing opinion on SS-Tube combinations. I seen a fair share discussion on this, and what combination prevails is the opposite of what prevailed before. Cases can be made for any combo, but the best combo at the ultimate price, imo, is tubepre w/ tube amp.

Labtec and Ohlala, brings up some good points. Many prefer to use a tube pre with a solid power amp. This is due to costs, heat dissipation and trying to make the most of the typical advantages and minimize the typical diadvantages of tubes and solid state. Everything posted so far is true, it's a big picture to view. As for running a tube CD player into a solid state amp, you usually end up with impedance challanges that limit choices of compatable ss amps. The advantage is a simpler less expensive signal path. Other disadvantages include limited selection of CD players and often limited or non-existant switching capability.

Others have made very good points about the relative merits and potential hassles of the configurations discussed. Another possible alternative is a hybrid amp, which adds (if properly chosen) tube qualities without the need for mating with highly sensitive speakers. Tube replacement/rolling, as with tubed preamps, is relatively inexpensive. My system currently has a solid state preamp with a hybrid amp and I like the synergy. I did later change to a tube DAC for a touch more warmth. There are lots of ways to achieve the synergy you're after.