Looking at the Stereophile measurements you should take care in choosing a tube amp as the Gold 300s look like a bit of a harder load than one might initially expect. Here’s an excerpt from John Atkinson’s measurements section…
Although Monitor Audio specifies the Gold 300's sensitivity as 90dB/W/m, my estimate was almost 3dB lower, at 87.3dB(B)/2.83V/m, perhaps due to the fact that the Gold 300's impedance is specified as 4 ohms. (An input of 2.83V is equivalent to 2W into 4 ohms.) The impedance magnitude (fig.1, solid trace) does stay close to 4 ohms from the upper bass through to the mid-treble, with minimum values of 3.7 ohms at 117Hz and 3.57 ohms at 1kHz. There is a current-hungry combination of 5 ohms and –39° electrical phase angle (dotted trace) at 77Hz, and the phase angle exceeds +40° above 10kHz, presumably due to the inductance of the MPD tweeter's drive system. The Monitor Audio needs to be partnered with an amplifier that is comfortable driving low impedances.
A “modest tube amp” might not get the job done here, so make sure to do your homework before fully committing. In this case you might consider going with a tubed preamp rather than an amp to get some of that tube goodness. Don’t mean to be a bummer here but thought I’d share this info in case you weren’t already aware.