I just got my MD-90 about 3 weeks ago. Couldn't be happier. Replaced Creek T43 which is also very good, But MD-90 edges it out. Can't comment on tube upgrade, have not heard it.
The MD tuners are beautiful tuners and I don't doubt you would enjoy it, but for $1000 I suspect you could buy a vintage tuner for a third the price, or less, and outperform the MD. Furthermore you could have the vintage tuner sent off to be modded and have an incredible tuner, still far under 1k.
Selecting a FM tuner is very complex and everyone you talk to has a different opinion. Please see www.fmtunerinfo.com for even more suggestions. I researched tuners for several months and people told me the Magnum Dynalab MD-102 was better than the MD-90. However, I did not want to pay $2,000+ for the MD-102. The vintage tuners were highly recommended but I could not find one in an acceptable condition (to me). I finally purchased the Meridian 504 FM tuner for $600 that included a 2 year warranty from Meridian. While it does not have all the control features, it sounds excellent to my ears. I am using the Fanfare FM-2G whip antenna. I hope this helps. cheers..
No. I was not able to audition the Meridian 504 against the MD-90 or the MD-100 (or the MD102). Retailers in the San Francisco Bay Area do not stock any of these models BUT, of course, would be happy to take my check to purchase one. I researched as much as possible on various audio sites and contacted several people who owned various models. The people that owned the Meridian 504 were very happy and for $600, it was a good deal. The 504 sounds excellent. cheers.