The crank up really depends on how loud you're going. Your receiver is rated 90W x 7 speakers. A lot of consumer level receiver do exaggerate their ratings. It could be a lack of power if you're getting distortion if you listed to all of that loud.
Again, a receiver is a pre-amp and amp all in one with the brains being the pre-amp and the amp being the power. Some receivers can actually act as both a pre-amp or a receiver. The types of customers who want a pre-amp typically spend a ton of money on AV, so companies like Pioneer don't make your model with the ability to do anything but be a receiver.
Should you actually need more power or want to have more power, you'd have two options. A more powerful receiver which again is the pre-amp and amp all in one or buy both a pre-amp and either a 7 channel amp or 7 channels worth of amps. In terms of an amplifier, if it's operating within its power rating then no one amp sounds better than the other. Thus, you can find one used for a good price. The pre-amps can get spendy as they're aimed at the high end market.
You could get a cheaper receiver that has pre-outs (as in it can be a pre-amp) and then buy amps if it doesn't do its job as you want. You could sell your existing receiver on craigslist or somewhere and buy a used one that has pre-outs. Then buy a used amp for whatever speakers are distorting. The center channel and front speakers do most of the load typically.