Best pre-amp to match with the Dynaco ST-70?

Try a passive like the Creek OBH-12 or, for a lot more money, one of the Placettes. Both give you remote volume control and are very transparent, impose hardly any sound of their own, especially the Placette. Any pre-amp will most likely impose a veil or "wash" of coloration. With a passive you will be hearing your amp directly. You'll need to check and make sure your CD player is compatible.

If you can solder, look at the Bottlehead Foreplay. It comes as a kit, just like an old Dynaco. Very cheap, only $149. I use one with an ST-70. It is much better sounding than the price would indicate.

I drove a dyna 70 w/ a Rogue 99
It sounded incredible!
I miss my old dyna (and 99)

And what is wrong with using the Dyna PAS 3X with the ST-70?