Pandora Stomp harsh sound from amplifier

[davidpoole]

My Pandora Stomp is great for my guitar hobby through headphones (Sennheiser HD490, and others), but very harsh on top strings through amplifier (80's Rowland Cube 40). Sounds like the strings are buzzing on the frets! Now trying a new Fender Acoustasonic 40 amp - just as bad. Tried reducing Stomp output, and Cab Through on/off, and obviously turning down the treble, still bad, even with another (new) guitar. Sounds better with a small hifi external speaker fed from the Cube 40. Wonder if the "whizzer cones" on the Fender amp (Cube 40 has something similar) are picking up high frequencies from the Stomp? No problem playing straight into either amp or with Stomp on bypass (but no effects!). Can anyone suggest a fix for the Stomp, OR a compact guitar amplifier that does give a good sound with the Stomp. Ps, I play most styles, but not heavy metal.
David