I have used a Hotplate with various different amps, and had different experiences with all of them.
With the AC30, it was quite a considerable tone sucker.
With the 50W Plexi clone, it worked really very well. With the PPIMV on the amp, as well as the Hotplate, I could get very, very nice tones at any volume.
With the Orange Rockerverb 50, I preferred the lower volume tone just using the MV on the amp, so didn't ever see a need to use the HP at all with this amp. The RV50 does seem to work exceptionally well with its MV though.
I sold the Hotplate. If I buy another attenuator it will probably be a Sequis Richter Control (£279 at Digital Village). These are only a tiny bit more expensive than a Hotplate but I have heard several absolute glowing reports about them. Even going as far as saying they are on a par with the Ultimate Attenuator (which is of course way more expensive to import into the UK).