Bob, could you explain why QSC makes lighter and heavier amps? I didn't go through all the specs of the two models I mentioned earlier in this thread, so excuse me if there is any huge difference I missed!
The RMX series amps use conventional power supplies, in which the power transformer operates at the AC mains frequency (50 or 60 Hz). Conventional supplies are inherently simple and easy to design, and they're pretty easy to scale up or down in power. The biggest thing is that as long as the transformer and reservoir capacitance are scaled adequately to the task, it's hard to go wrong. The RMX circuit designs are largely based on past QSC amp ranges like MXa and EX, only updated.
The PLX amps use switch-mode power supplies, in which the power transformer operates at a high frequency (approx. 110 kHz). The supplies are much more complex and involved a lot of R&D. Essentially, they take the incoming AC, rectify it into DC and store it in a big reservoir capacitance, and use that ~340-volt DC to make 120 volt 110 kHz AC for the transformer. The audio circuitry is a bit more elaborate, too, because Pat Quilter designed in a bunch of little circuit tweaks to address typical power amp issues such as crossover distortion, accurate thermal tracking of bias, et al.
Both lines are general-purpose power amps, as opposed to our power amp lines that have features specific to certain market segments like touring, cinema, installed systems, etc. QSC contracts with a company in China to manufacture the RMX amps, but the PLX are made right here in California in the same building I work in. The prices of the four lower-power RMX models are much lower than the PLX models.
I usually advise someone trying to decide between RMX and PLX that if you're gigging maybe once or twice a month, go with RMX; the weight won't bother you that infrequently, and you can make the financial return on your investment more quickly. If you're gigging a lot, like once or twice a week, you should consider the PLX because the lighter weight will probably be less wearing on you, and it would probably be a lot easier to swing the higher price.
That said, I won't tell you that using any particular brand or model of power amp--even QSC--will give you superior sound and tone over another, because it almost always isn't true. Look for brand reputation, reliability, support, service, etc.
However, always interesting to me... especially when discussing 'all in one heads', which is a little less 'dry' than pure power amps
