New build works but the sound is a little percussive?

[Lectroid]

Hello all,

I've just finished my first build.  It's a conversion of a Bell & Howell Model 179 projector amp--see schematic below.  Turned out better than I'd hoped, for a first effort.  The sound is clean at low volumes and breaks up nicely at higher volume settings. There's a little hum but no more than typical 50's era background hum. The only problem is that the sound is a little percussive to my ears at low volumes.  If I turn it up, the sharpness fades away and the sound breaks up nicely with a warm tube sound almost like my Marshall JTM30.  But playing at lower volumes, the picking sounds sharp to my ears.  Guitar is a Les Paul Studio.  The tone circuit is the typical Fender 5E3 arrangement that interacts with the volume.

Can anyone see any reason in the circuit why this might happen?  Any similar experiences?