Using a preamp with my Ashdown ABM head...help needed.

[jcburn]

Hey guys - I have tried a few times to hook a preamp into the effects return of my Ashdown ABM500 EVO III (to bypass the amps own preamp) only to have no signal come out. I thought it may have been a fault with my head but thought no more of it.
Was at a music store over the weekend and again tried to use an Ampeg BSP and Aguilar Tone Hammer preamps the same way through a store ABM500 - same deal. The store guy was perplexed too.
Is this just how these amps are?

 

[René_Julien]

Have you tried the Line input instead of the effects return?
IIRC Ashdown treats the effects loop differently than how it's conventional.

The Line input bypasses the preamp I thought.