In you decision making process, be reminded of the following facts:
- Going from a 150 to 300 W amp, you will gain 3 dB increase, and will sound louder just like turning the volume up by 3 dB, or 6 clicks if the volume increase is the typically 0.5 dB per step on Marantz AVRs.
- Doubling seating distance will result in 6 dB loss assuming no room gain, so if you sit 4 meters instead of 2, you will lose 6 dB SPL, like turning the volume down by 12 steps (again assuming 0.5 dB/step).
- "Power", while it is mainly a product of V (voltage) and I (current) can be misleading in many cases, it is better to pay attention to power amp's voltage and current capabilities instead of just focussing on their advertised "power" specs in watts. A power amp's output of say 150 W into 8 ohms, may be okay outputting 200 W into 4 ohms, but if there are lots of contents that require the amp to drive long enough into 4 ohms and below, it will not be able to sustain at that output level.
That said, I would never use those bass X series amp with large not too easy to drive speakers like the A9 unless for near field and required SPL is not particular loud. Polk spec them as 8 ohm nominal, but the actual impedance/phase angle curve would likely tell a less optimistic picture, do you have those curves, if do please post so we can something more definitive of their "power" needs.
