BBIBH is quite correct. Unlike transistors, tubes begin deteriorating from the moment they're first turned on. As they age, they may become microphonic, "gassy", and introduce all sorts of unwanted distortion and hum.
Capacitors in tube gear also age, so be careful with your new M40s when you fire up the old tube Onkyo.
On the plus side, even if the Onkyo is low-powered, tube amps tend to clip quite gracefully, without the harsh distortion associated with some solid-state amps as they approach or go into clipping. If you have some old speakers around that you don't care about I'd first try the Onkyo with those, then, if don't hear lots of noise, crackling, hum and the like, switch to the M40s.
Axiom Resident Expert