Whether it is optimum or not is ultimately determined by your ears, your expectations, and your speakers (not listed). I agree it sounds like an adequate match on paper but there is much we do not know.
I would forego the pursuit of impedence matching devices and wait until you have done a lot of serious listening to the basic set up with your speakers. If it ain't optimum to you then consider if the sound's inadequacy is caused by an impedence mis-match or something else. It could be corrected by something as simple as a tube selection in the pre-amp (isn't that why we like tube amps/pre-amps? :-), and not need impedence matching devices to get you where you want to go.
FWIW
I would forego the pursuit of impedence matching devices and wait until you have done a lot of serious listening to the basic set up with your speakers. If it ain't optimum to you then consider if the sound's inadequacy is caused by an impedence mis-match or something else. It could be corrected by something as simple as a tube selection in the pre-amp (isn't that why we like tube amps/pre-amps? :-), and not need impedence matching devices to get you where you want to go.
FWIW