I will remain silent on the load box stuff as I cannot contribute to that topic as I no longer make use of attenuators for recording purposes.
FWIW, as long as you have healthy power tubes that aren't TOO cold or TOO hot, I'd recommend focusing on your speaker choice instead. The speaker & the microphone choice are the top 2 determinants of tone.
I like my mic'd speaker sound, but I'm in an apartment and do my best work after midnight. So I captured IRs of my speakers with the mics I record them with. 4 speakers in a stereo wet/dry/wet config, a 2x12 on the Mk V for dry (after FX loop EQ) and a stereo solid-state 2x12 for stereo wet FX. 2 mics/speaker (8 total).
I recreated the rig with IRs (8 IRs in place of the 8 mics). The dry 2x12 gets the Mk V slave out, the wet 2x12 gets the outputs of two load boxes on the stereo amp.
When speakers are connected to the Thru of the load boxes, the slave/line outs -> IRs sounds very close to the real mic'd speakers, but when I go silent and switch to using the internal dummy loads, the mids go flat and the Suhr and X-Load UK voicing get REALLY boomy and mid-scooped. Tbe X-Load US voicing and TN Captor aren't so bad, but they still go flat in the mids (lose that 3D depth to the mids that's still there thru IRs when speakers are the connected load). Then the Rivera Mini Rock Rec sounds really flat and boxy, almost all mids, not too different from the flat sound you get from plugging the FX send -> IRs.
The Suhr is the worst: way too bass-heavy and scooped, but only on the Mark V, not the solid-state power amp, so I'm thinking it has a lot to do with how it's reacting with the tubes. The Suhr RL seems to be favored by the majority
I know tubes won't change that much, but I need a new set soon anyway, so I'm trying to put a lot of thought and research into which ones I get. Mesa website says power tubes are at their best 1.5 years at 10-15 hours/week... I use mine 30+ hours/week. I have the 12 year old 440's that I've had for 4 months, and a quad of JJ's that have been used full-time for 6 months. They were newer, and the Mk V had all kinds of noise and signal-loss issues when I got it, so I put the JJ 6L6GC's and all new preamp tubes in it. And I need to put one of these sets back in my other amp, I just took the 440's out of it to compare to JJ, and the JJ's have a more pronounced harshness with less focused mids.
I haven't been able to find any videos or feedback about the Powerhouse vs any of the 4 loads I have, so I'm reluctant to spend $470 on an open-box to find out if it might actually be significantly better than what I have. Because I don't need an attenuator, or IR loader or anything, so I've been avoiding the $6-700+ boxes. I need at least 3 for the w/d/w setup, and sometimes put two 8 ohm loads on the Mk V to see what mixing them does.
The weakest link in my chain is the loads, because I like the mic'd speaker sound and the line out -> IRs sound when speakers are still connected. But I need silent recording for the apartment.