With that interface and the attenuator - I can't see any advantage at all in an additional device in the signal path. Your interface is designed for guitars, and the attenuator has the same design purpose - connect the two. You could try daisy chaining them, but unless you go through the faff of having two active interfaces, I doubt you could swap quickly enough to hear differences without memory creeping in to mess up the comparison. Electrically - no issues, but practically, I'd personally not bother.

RichD, post: 463018, member: 51808 wrote: I recently bought a Universal Audio OX attenuator for recording guitar on my tube amps. Does anyone know if I can still use my Focusrite ISA One pre in between the OX and the Apollo Twin Duo digital interface?

Assuming you have the Mk II version of the Apollo Twin Duo, you could indeed take the analogue output of the ISA and send it through the Apollo in order to make use of the UAD emulation plug-ins in the Apollo. However, as Paul points out, this would be sending the signal through two pre-amp stages, which may degrade the signal quality slightly.

If your ISA has the optional A-D converter installed, a better signal chain for recording and/or making use of the effects would be to take the S/PDIF output from the ISA and route that to the optical input of the Apollo, by-passing the Apollo's pre-amp and converters.