Indeed -- I was immediately interested in this topic, since I have both of those devices (plus others). The DX21 was the first. Whenever I think I'll sell it, I play it a bit and change my mind.
There's a psychological aspect here: I figured the DX21 as a more affordable DX7 ... so when I got a DX7ii, I expected the DX21 would have no more interest for me. (After all, having more operators is "better" right?). But the DX21 has that one slider -- you know, the one which lets you mix the two layers -- which somehow proves impossibly addicting for such a simple feature.
In both cases I got them because they are quality-built machines: 35 years old, built like a tank, suitable for personal defense in the event of a rowdy audience.
Back to the topic: I expect the DX7iiFD would do better talking to a TX7?? I got the TX7 after reading comments disparaging the DX7ii in comparison to the original, so figured that way I'd have both worlds. Haven't tried layering them together yet -- been too busy climbing other learning curves ...
