I use an ST-150 with Vienna Acoustics Beethovens and I concur with Jc1373 on its sound characteristics. The mid range and upper end are exquisite. The bass is a little flabby in my systems but the Vienna's woofers fall just outside the recommended range on VTL's web page System Matching section. They suggest using the ST-150 with woofers no bigger than 8" or 2 6". I use the ST-150 almost exclusively in tetrode mode as I listen to mostly rock and electric blues. Triode sounds magical with small string ensembles or solo acoustic material but doesn't have the drive I want for rock or full orchestral works.
Cheers,
Jim
Cheers,
Jim