Is no preamp really better that a good preamp?

All true Phusis, but the OP was asking specifically about adding a separate (perhaps tube) pre-amp between both his DAC & phono stage and his power amp.

I submit the answer depends on the components involved. I had a PS Audio DSD DAC and compared to the Parasound JC2 preamp, I initially preferred the DSD direct. After extended listening I felt I lost dynamics and detail. I sold the DSD and Parasound JC2 to purchase a Bricasti M1 DAC. In my system the Bricasti was a definite improvement over the DSD. The dynamics were there, detail, but I still felt like something was missing. I went from the Bricasti M1 directly into my Jeff Rowland 525, to the M1 going into the Jeff Rowland Continuum S2 integrated. It sounds better than ever. I don't know if it's because the amp section of the Continuum is that much better, the added preamp section, or a combination of both. I prefer music with a preamp.

A great preamp is the heart of a great system.

The burden of proof should always be on adding anything to the signal path that is not an obvious necessity. In this specific case a preamp is not an obvious necessity, and per George's analysis (with which I agree) the odds appear to be in favor of that burden not being met. However, as others have indicated the only way to know for sure is to try it.

Good luck. Regards,
-- Al

A P.S. to my previous post: If you do eventually have an opportunity to try a tube (or other) preamp in your system, and if you find the resulting sonics to be preferable to no preamp, keep in mind the possibility that the root cause MIGHT be a less than optimal tonal match between your power amp and your speakers. Particularly in view of the impedance characteristics of the speakers (4 ohms nominal, 0.8 ohms at 20 kHz), which could conceivably result in an over-emphasis of the upper treble in conjunction with some solid state amplifiers.

Should that ever prove to be the case, changing power amps would probably be a better solution than adding a preamp.

And if there is any uncertainty in that regard, researching reported user experiences involving the use of Spectron amps with Martin Logan speakers may be helpful.

Regards,
-- Al