I would also assess using 2x 306 instead, using one channel of each amp per speaker. The 306 has a
virtual earth type psu per channel, which needless to say is a good invention of theirs, but stops you being
able to use both channels of each amp per speaker, rather one channel per speaker, gives nice advantage
for audio reproduction

If you choose the 909, you will need to spend an hour or so applying good quality heatsink compound,
(with cautions to directly touch the compound ) to assist transfer from the transistor aluminium bracket to the outer fins,

Either amp, suit passive attenuators including opto coupling and are very good sounding, and will be a excellent choice with the 63';s