What is a better route, tube or SS preamp?

If you believe that a tube preamp or amp sounds better, then it will. The same goes for Solid State. Both types of electronics are a means to an end. In the end it comes down to personal preference and affordability. Its funny how in the 80s to early 90s good tube gear could be had for a fairly reasonable price that sounded as good the SOTA, very expensive SS gear- albeit with some component upgrades such as better wiring, capacitors and so forth. Now, it seems both types of electronics have comparable pricing. Tube gear has an ongoing cost that must be considered- replacement tubes. Not too bad when it comes to preamps, but big power amps can carry a significant maintenance fee for the periodic replacement of tubes.

I have actually heard a Naim amp that a friend had that we attempted to pair with tube linestages. The result, with several models was not that satisfactory -- sluggish, murky sound. Of course it might have worked well with a different linestage. As with any kind of matching, one has to actually try the particular combination to know if it will work well. From my experience, the least predictable match is a tube linestage/preamplifier feeding solid state. I am not saying it would be bad, but, I have heard enough combinations that were not good so I would say the results are highly unpredictable. The other way around is much more predictable -- the particular characteristics of the solid state linestage seem to carry over more predictably when coupled with a tube amp (this may too be a disappointment if one had hoped to somehow radically change the sound).

The other thing to consider, when going to tube feeding solid state, is whether you intend to keep the solid state amp on all of the time (usually helps the sound and is, I believe, the recommended practice for Naim amps). If that is the case, you will be switching on the tube linestage while the main amp is active. A tube amp being turned on, and some when being turned off, can emit pretty nasty noise. Some tube gear cures this problem with a mute circuit that either automatically mutes at turn on, or requires manual muting until the unit is warmed up. This is not an issue with all tube systems where it is common practice to turn the amp on last and off first.

I don't know if a tube linestage is more likely to produce dangerous, speaker threatening impulses. I have actually experienced more problems in that area with solid state gear myself. the primary maintenance issue with a tube linestage is aging tubes that will cause deterioration in sound quality (typically making the sound dull and lifeless) or result in increasing noise. I would have an extra set of tubes ready for replacement and "testing" (every year or so, try putting in the replacement tubes to see if the sound of the original set has deteriorated enough to warrant replacement). Apart from tube replacement, tube gear tends to last quite a long time and is repairable (solid state gear is more likely to become unrepairable because of unobtainable transistors and ICs).

A few serious technical facts...tubes glow, you can see into them, they make you feel good, and thus are better. You can't have too many tube devices in your system as there is no limit to how good and better you can get.

Problem with tube., A. half the NOS ones you buy as "testing great" are noisy and at end of life.
B. the other half are fake.

Schubert is correct about NOS, but don't let that scare you away from tubes. Current production is improving rapidly and if you must have NOS, buy directly from an established dealer and/or buy a tube tester.