Just purchased an new Mustang IV. Only thing missing from my wish list of requirements was a built in looper. Any suggestions out there for a suitable external looper that would be compatible for the new line of Mustang amps? Thanks.

I'd suggest a Boss Loop Station. There's a dual pedal version which is more expensive, or a smaller more affordable one with most of the features. They work great with anything.

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Jamman Solo?

Or JamMan Stereo. It's got a ton of features including the ability to use SD Memory Cards so you can save even more loops. Not sure what you want? Both Boss and DigiTech have PDF versions of the manuals available for download on their web sites.

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"I don't trust nobody, not even myself" - Willie Dixon

I use a Line 6 JM4 Looper guitar effects pedal with my Fender tube amps and it works great. All types of effects along with bass rifts, drums and songs. I would suggest you head over to one of the big box music stores and try several different loopers out to see which one you like the best.
Kenny V

What about the Fuse software that came with the amp?