I'd like to hear some opinions.
I'm not a guitarist,..But I've heard this is what my guitarist
would need for retaining the sweet spot sound at lower volumes,
when tryin' to add more volume.
I've had an 8 Ohm Hot Plate for, hmm... 3-4 years now? I've used it with a Mesa Nomad-45, a Rect-o-Verb 50, and a Rectoverb Roadster over that time.
Does it do what it says it does? Yes, mostly. But some of that is going to depend on the amp more than the Hot Plate, and where the "sweet spot" falls. The Nomad actually sounded pretty damned good at low bedroom volumes, and to be perfectly honest it didn't NEED the Hot Plate, so any improvement in the tone was pretty marginal. And the Roadster, while definitely a little more volume sensitive than the Nomad, also held its own pretty well at bedroom levels, so if I didn't already own it I probably wouldn't have bought it. It helps a little, but it's not a night-and-day improvement.
The Rect-O-Verb? Damned near unusable without the thing. For some reason the power amp REALLY colored the sound of that amp. At low volumes it was bright, spikey, and fizzy, and as the volume came up the power amp got
seriously dark and smooth, and really radically filled out the sound. The problem is, when I say "as the volume came up," I mean the channel volume would be at 9-10 o'clock (sweet spot for leads, IMO) and the master would be pushing 1 o'clock, which even for a 50-watt Mesa falls somewhere between "Somebody call the fucking cops!" and "Good lord, somebody call a preacher, the Rapture is upon us!!!" Running the Hot Plate at -16db attenuation brought that down from somewhere past absurd to at least at a level where my neighbors probably wouldn't declare war.
Did it color the sound? Yes. Did it color the sound more negatively than the extra power tube saturation colored it positively? No. For that particular amp, it made a ton of sense. For my Roadster, it's less open-and-shut, but I kind of like how it takes a little bit of the edge off the high end for leads. For my Nomad, there was really no need.
I'd say see if you can get a shop to loan you one for the afternoon, or barring that bring your rig into their live amp room and try it out. I'd imagine it'll help the 5150 quite a bit, the Triple a bit, and the JSX a bit less so, but I have no first-hand experience with either.
Also, be reasonable about your expectations. It's not going to give you kickass power amp saturation and "amp at ten" tone at whisper-quiet volumes. That's not possible. What it CAN do, however, is give you the level of power amp compression and saturation you'd expect from a couple notches louder on your output knob than the volume you're actually hearing in the room, which sometimes can be a very good thing.