I don't mean to knock the Bottlehead preamp but the one that I used one time (IMHO) had too much gain. It was equipped with two 11 step attenuators (way too few steps !) and you only had about 4 or 5 steps before the amp was pretty loud. I don't even know why they would sell a preamp with only 11 steps of volume ? Maybe your Bottlehead preamp has different attenuators or maybe they have changed the volume attenuators in more recent Foreplay's.
One other issue might be that the VTA ST-70 with the standard gain board with 12AT7 driver tubes does have a little more gain than the average tube amp. If the VTA ST-70 doesn't hum with another preamp then it's not the amp. If the Foreplay works OK with another amp and does not hum, then maybe it is the interaction of the two that is the problem. The VTA ST-70 and ST-120 have a 270,000 ohm input impedance and work well with virtually any preamp.
If they won't work together properly, then my choice would be to sell your Bottlehead Foreplay and consider in its place the PAS-3 Clone preamp KIT which IMHO is a much better preamp with significantly better bottom end response than the Foreplay.
VTA PAS-3 clone preamp kit