I have GY Silent Armours after MTRs, but as the harshness is revs-related (occurs in each gear at the same revs), I doubt there's any connection with the tyres. Thanks for the thought anyway.
I haven't yet run a belt around the gbox soundproofing, although it really doesn't seem to be something light vibrating. The harmonic balancer wobbles at idle, but at almost $600 for a new pulley I wont be changing that without expert advice.

