Newbie Tuning Help - Helix P6 (AutoEQ/REW), Properly Setting Gain/Output Before Tune

Holy crap am I glad I found this post...my long-delayed F150 install has come to this point recently and I wasn't sure what to do downstream from my DSP. This helps tremendously, but of course I have questions.

I have 6-channel analog coming out of my LOC (Amppro FD21) into a Helix DSP/amp (M4)...first 4 are full-range, last 2 same but through a 'bass knob' attenuator. The M4 amp pushes all four doors currently and it processes 4 more low-level channels...3 to a Key 2004 for tweeters and center, 4th is a mono sub channel split to a Key 5001. Source is the OEM HU, primarily my ESS Sabre-equipped phone via 3.5mm AUX.

I have access to an oscilloscope and started checking things recently. Confirmed ZERO clipping at max output, first on the phone output, then on the LOC output. I started with the LOC set to "4V" output (noise floor paranoia), and with 0dB 1kHz (channels 1-4) and 40Hz (channels 5-6) sine, max volume on phone/HU/bass knob, I saw a 3V (+/- 0.1V) perfect wave into the Helix. But, it was quiet. So I put the LOC back on 5V output and now see 4V, still stable and clean. Helix (single) input gain adjustment is set to 4V low-level, all output gains (amp or low-level) are currently at 0. So far so good, right?

Next I went to the sub output, as access to that one was easiest. Same max-everywhere 0dB 40Hz sine shows me a hair under 5V into the amp. Key 5001 inputs are low or high on the same wire, spec'd up to 10V. If I crank the Helix output, it's clean on the scope as high as it will go (don't recall the peak V)...but I'm still at 0 for now. Now, with amp gain to zero and the scope on the speaker output, I've got ~25V clean wave. Increasing the gain, the peaks start to get a little fuzzy at maybe 42-43V, and there is clearly some clipping on the back side of the peaks starting above 45V. The gain knob is barely moved at this point (under 20% of the max for sure) and the clip light doesn't so much as flicker, but the scope shows bad wave form.

Questions here: testing was done w/engine running but NO load resistor. Does that junk the results? 45V into my 2ohm sub load would is 1000W...that ain't right. I'd like to set the sub with the attenuator near 50% (so I can cut/boost). I'm thinking next steps here are to put a 2ohm load in the circuit, set the knob where I want it, and look again? With 0dB I should have gobs of headroom to twist it. Or maybe use -5dB? Amp is rated 300W @ 2ohm, so that's ~25V. Concerned about having enough headroom to use the bass knob some (hardly a bass-head w/300W into a single sealed 10). High input, low gain is good...I'm barely into the gain as-is, but the Helix can output more and the 5001 can take it. Should I put the amp at zero and use the Helix to boost? Finally, bad idea to use the actual sub as the load right?

And then I punted. The other amps are mounted in such a way that access to the speaker terminals is challenging without removing the rack. Plus a couple short-term medical issues, and working under the seat is just an impossibility. But this! Jdunk says do the highs by ear for sure, and I will. No signal, crank the software output gains one by one until noise, set all a hair below the lowest value...easy for the Helix amplified channels. Do I have that right?

What about the other amp? Should I be looking at the input signal there at all? I expect it will boost as cleanly as the sub channel and the amp inputs also good up to 10V. Should I try to fine tune voltage (around the 0dB default) with a sine, just to level-match...since I can? Then, same process as above (using the amp gain knob), and the same question as w/sub amp...better to be into the amp gain a bit, or zero and noise-hunt w/input gain? 2 gains (for channel pairs)...if I have 2 drivers on this amp that need significantly more power than the 3rd, would it be wise to wire such that they are not on the same pair?

And if you've read this far...what's with the shitty "400W" 120V inverter in the F150? My laptop PS says 1.5A draw, and the truck won't push it.