I've had couple DD55's in the past, and they sound as good as whatever you run them into with the midi out.
With a synth module with drum sounds, or sampled drums in a computer they sound good. The stock sounds are not so good, but IMO, acceptable for what those units cost.
The biggest issue is probably the size of the pads. The smaller pads are about 3 inches across, if I remember right, and require a bit of precision when you get flying.
I'm surprised to see that Yamaha has dropped the midi out connector on the 502. I suppose it's to be expected with all the usb stuff nowadays, but disappointing for me because I usually use regular midi ports to trigger synth modules from pads.
presence - what do you have now? If you just have the dd65, I'd say instead of buying another module and pads, maybe you should consider selling the DD65 and picking up a Yamaha Multi-12 unit.
Probably come out close to the same cost when all is said and done.
For a few more bucks for pedals, you could have a setup something like this: