hi clive101, i think the basic message in Paul's video (above) is - if one has a choice in amplification (in terms of watts), go with the larger amp because it provides more headroom and - i believe - deliver lower frequencies with more precision. But that's just me.
In terms of active vs. passive, each of your 7B amps is delivering up to 600watts into each of your PMC passive speakers. The internal crossover network inside of each speaker is directing a unique set of frequency ranges into each driver (one set of frequencies into tweeter, mid-range, and bass driver). In the passive set up, the incoming signal to the crossover has already received the full amplification from your power amp.
In the active version of your speakers, the incoming signal is split into 3 sets of frequency ranges before amplification. Each unique set of frequency range is delivered to each of the 3 amps (one amp for high, one for mid, one for low)....then each amplified signal is delivered to each driver.
The power levels you mention (325w, 150w, 150w) correspond to the power of each dedicated amplifier in the active version of your speakers. Each power amplifier in the active setup is dedicated to a single driver. (low, med hi). So in total, the active set up has 6 amplifiers - corresponding to 6 drivers.
Since a 325w amp is used for the low frequency driver in the active setup, a person could interpret that a 7B would be more than enough power in a passive setup. But that 325w is applied to a more narrow range of frequencies for only one driver. A more efficient power delivery system.
Ultimately, to make a decision on a 28B vs. 7B i really think a person would need to A/B test both amps in the same listening conditions. Or....to be on the safe side....just go with the 28's
Hope this helps.
TJ