Power amps sound different because the manufacturers use different components, and different designs (class "H", etc) in different amps. Even among the same brand, different series of amps will sound different. I have QSC "EX" series and "Power Light" series, and I can hear the difference between the two, especially on bottom end (I like the Power Lights better, but the Crests still sound better than that). Durability is another factor that separates amp manufacturers. The big amp names (Crown, Crest, QSC) are the big names because they have both sonic quality and durability. After you get your rig dialed in and you get used to it, see if you can rent one of the big name amps for a weekend, and see if you can tell the difference. I would rent a sub amp, because I personally hear the biggest difference in amps on subs, but that is just me.
HTH!