I just bought an AC15 Red Limited Edition and fell in love with the sound.

But I played my Vox AD30VT modeling amp on the AC30TB model and it sounds just like my new AC15.

Should I just play with my modeling amp set on AC30TB and return my new AC15, or should I keep the AC15?
I think you should play them a bit more and make your own decision.

We can't really tell you if you hear enough difference to justify one amp over the other.
The AC15 will respond to picking dynamics far, far better than the VT. If you can't hear the difference I'd say that the problem is in your fingers.
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I have a decent amount of experience on both amps, (I own a VT30, a friend has an AC15), and I don't think you're using the AC15 to its full potential, or anywhere near it. With a lot of limitations on the AC15, the vt30 will do an admirable job of modeling the AC. Thats why they're recommended so much in new player "what amp" threads, they actually sound pretty good and are better than most, imo, for that purpose (newb guitarist). Have you tried playing with a drummer while comparing them? That's when the differences start to show. The AC sounds better the more you turn it up, the VT reaches a point and it starts to get a lil harsh and muddy. Are the tubes hot? Are you aggressive with the guitar?

I love the Valvetronix, its a sweet amp for what it costs, but I'll bet you'll gradually hear the differences.
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