Stickman,
It is not surprising that a stand-alone phonostage, particularly one as expensive as the Ref. 2, improved upon the built-in phonostage of the Hovland. Not that many built-in stages have impressed me that much (the Audionote M-8 is an exception, albeit a VERY expensive exception).
Sorry to hear that you had bad results with the Mullards. I've heard Mullard 12AX7s in my phonostage (Viva Fono), a Tron Seven and the HP-200. I did not particularly like the Mullards myself. In the Tron Seven, in particular, the Mullards sounded very strange and phasey.
Given the price of some of the rarer 12AX7s and 5751s, I can understand a reluctance to experiment, but, I think that with something as good as the Hovland, it is worth the effort. For a warm, rich sound, Amperex Bugle Boys are worth looking into; for a more dynamic, but leaner sound, I like Telefunkens. I run the Telefunken ECC803S in my phonostage (lean, with extended highs and extremely dynamic), but these have become exceedingly expensive.
I agree with Topoxforddoc that the Tron is also worth looking into. I know that the designer insists that the supplied tubes provide the best sound, but, to me, that is a matter of system matching and taste. I thought the Amperex Bugle Boys sounded the best in the Tron Seven in the system I heard (MUCH better than the manufacturer supplied tubes). I think it is a big advantage of tube phonostages that the sound can be optimized by changing tubes.
