I MAY have to try a pair just to satisfy my curiosity ... though I am not sure I can drive them with 6w of tube power. Might have to break out my old Yamaha 2020 receiver!Keep in mind that the deep bass boost provided by the 901's external equalizer will eat up a lot of your amplifier's power capability, if the music being listened to has significant content at low frequencies.
Based on a quick Google search, it appears that frequencies in the 30 to 40 Hz region are boosted by amounts between 12 db and 20 db or so, depending on the setting of the bass contour control, and perhaps also on the version of the 901. With lesser but still significant boosts being applied up to well over 100 Hz at the control's center position.
A boost of 12 db would in effect reduce your 6 watt amplifier to a 0.375 watt amplifier on notes in the corresponding frequency range, while a 20 db boost would in effect reduce it to a 0.06 watt amplifier on notes in the corresponding frequency range.
Regards,
-- Al