For the OP, the bottom line is you're OK using Ni-MH batteries, but for purposes of fun discussion and battery banter...I'm gonna nit pik your response a little bit. In the end, it really doesn't matter though.
I agree with your comment about how the flat part of the discharge curve is flatter on NiMh batteries than Alkaline, but when you check the details you'll notice that NiMH batteries start at a lower voltage than Alkaline. You'll also notice that, as you mentioned, when the battery gets to the end of it's usable range, it discharges to zero very quickly and the curve become almost vertical. The alkaline curve certainly discharges at a quicker rate than when it's on the flatter part, but it's doesn't go to a near vertical slope like the Ni-MH battery does.
When you have a six pack of AA's, a tenth of a volt on a single AA cell, means the starting voltage on a pack is going to be six tenths higher on the alkalines that the Ni-MH. The flat part of the curves are also going to be higher by six times the difference. If the pack is 2 or 4 cells, there is less voltage difference on the flat part of the curves between Ni-MH and Alkaline packs. If you superimpose the curves, it might be a number of hours before the voltage on the alkaline pack decreases and crosses below the voltage on the flatter Ni-MH curve. So, at that point yes the Ni-MH voltage might be higher than the Alkaline, but then at the end, the vertical cliff of the Ni-MH will again take it below the voltage of the Alkaline.
Taboot, if your ni-MH batteries are not new, they will have a higher internal resistance which will cause the flat part of the curve level out even lower...and again when you multiply that by six times, the difference between the flat parts of the curves widens even more so that the crossing point of the two curves goes even farther out to the right.
To be honest, it's really not a cut and dry winner on either side becuase the run time on each battery is all dependent upon what is the voltage cut out setpoint on the preamp. If that setpoint occurs at a point where the Ni-Mh may happen to be at a higher voltage than the Alkaline, then the Ni-Mh would be the winner. If the setpoint is at a point on the curves that's farther left or right from that point, then the Alkalines might be the winner.
Anecdotally, the phenomenon I've described above was shown by people when we were testing out the Tascam DR100MKii when it first came out a couple of years ago. People were discovering that they could run their units without problems as long as phantom power wasn't on. However, if they swtiched on Phantom, there was a high current draw and, with alkalines they could get maybe an hour and a half of run time before the low battery cut out was reached. Using Ni-mh, especially older used ones, the run time was 15 to 30 minutes.
(Don't be misled by the above, the run time SHOULD be much longer...the reason for the really short run times is that the cutout setpoint on the tascam is set too high. Tascam still hasn't fixed this through a firmware update.)
What it showed is that the lower voltage setpoint was reached much quicker using the Ni-MH than the Alkalines...which I think was a result of the fact that the flat part of the curve on the Ni-Mh occurrs at a lower voltage...particularly when you have 2, 4 or 8 batteries providing 3, 6, and 12v nominal voltage.
Yes, the PSP-2 does run fine on NiMH batteries. However, I don't believe the low power light will come on sooner. NiMH chemistry has a flatter discharge curve than Alkaline, so if anything it should come on later than Alkalines. In fact, it might not even give you much advanced warning at all since NiMH batteries maintain a stable voltage until they are almost entirely discharged, at which point the voltage rapidly falls.
Here is much a better explanation of this than I can provide:
Low internal resistance allows NiMH cells to deliver a near-constant voltage until they are almost completely discharged. This will cause a battery level indicator to overstate the remaining charge if it was designed to read only the voltage curve of alkaline cells. The voltage of alkaline cells decreases steadily during most of the discharge cycle.
Regarding current draw, although I have not explicitly measured this on the PSP-2, I cannot possibly imagine it's pulling 350mA without phantom on. I've been able to run the PSP-2 literally all day on 6xAAs (easily 10 hours) with the phantom on and there's no way it would last this long if pulling 350mA, let alone 500mA?
I'll test current draw when I get home, but I believe it's going to be much closer to 200mA w/ phantom on. Bottom line, I know it will last far longer than 4-6 hours and even running it all day at Mountain Jam this year, the low battery indicator never came on at all.
This makes sense. For clarity, when I stated 350mah I was basing this on the sample run-times I was getting using several batteries that I have of known capacities. I have a smart battery charger, so what I did was discharged the battery with the PSP2 connected and then recharged the battery with my smart charger, noting how many mah it took to recharge. I did this three or four times and got around 300 to 310 mah, so 350mah I was using was conservatively high for purposes for calculating runtimes. Obviously, whenyou recharge batteries, there are some inefficiencies from what goes in versus what you can get back out. I always knew this, but could never measure what the inefficiency was...so for purposes of making sure I had enough battery power for festival weekends, I always knew that the 350mah number was conservatively high.