what they were referring to; poor ground, or the alternator cant keep up with the new power.

the alternator may not be at fault per se, the battery may be insufficient. so it is either or both of ANY/ALL of these [if the noise is coming through the speakers while the amps are on]:

1Battery
2Alternator
3Amp wiring [poor connection, poor emi suppression aka did a $H1TY job installing it]
4Main Ground cables
5Circuit used to quiet the power signal to the amp of any normally present alternator electrical noise.
6Someone/something is underneath your car car being draged[for the love of god, look under and see!].


EDIT Just saw second page. ok, then the noise you're hearing is not from speakers, you were not 100% clear since, even though you seemed to say you turned off stereo, you still heard it and you blamed the stereo guy, so I got confused by you.

then the noise is probably : bearing noise [multiple places for that] , belt noise, throttle plate(s) noise, air intake [hose] leak, or vacuum leak. Of these , throttle plate/normal noise-- it's kinda funny you would be so sensitive to it. most v6 and better I've been in make it at higher rpm sometimes but I always thought it to be a sound of high performance. You must be really getting on that gas pedal!