Did you reset your gain with a dmm after switching to a higher HU voltage? I would imagine you didn't, which means your new setting are louder since you HU voltage increased, but it's no different than the result you would have gotten by turning the gains up a little higher on the old HU. All the HU's voltage does is get increased at the amp level, where you start has no effect on volume all that matters is how much power the subs see and that's on the output side of the amp not the input side. The gain is there for just that reason, to make the amp more or less effecient at increasing the voltage. (not effeciency in the normal sense here, just higher gains means less voltage in more voltage out, comparing to a lower gain setting.
To put it into perspective my ARC XXK amps which when they were new were very expensive (like arc SE series now) and are still well respected SQ amps. They only accept a 1volt signal from a preamp before clipping. Even using a 4volt HU, they've never clipped audibly, even with gains down so HU could go to 35 (max) and preamp clipping is generally easier to hear so if I was clipping the ouput of the HU and the amp input, it should have been very obvious and people with Pioneers have never reported a problem either. Just like with an amps output, unless your running 0db test tones, you'll never see the rated voltage from a HU source, 99% of the time you'll be well under 1 volt of power, even on a 8volt pioneer. Just some food for though. Pick a HU for it's other features, EQ ability, low pass, high pass, fully active capabilites, # of RCA outs, optical out, even the color before you pick it for it's voltage. It's a somewhat useless spec in most cases, but since it can be numerically defined, it's a easy thing to slap on a box and say "mines better than yours", marketing.
