A large percentage of the DAK club members here in Phoenix use Spektrum radios with their Tamiya tanks. I even got my Polk Tracker III 8-channel transmitter (an odd 75Mhz digital radio usually used for submarines, which is why I have one) to successfully control several Tamiya Shermans. I did have to adjust some of the digital control settings in the radio's computer to get it to work correctly. But after that was done the tanks ran fine.
Tamiya may have said one thing, but I have seen these tanks controlled with my own eyes using Spektrum radios, so it definitely should work.. Do you have anyone local in your area that has a ground radio that could be tested with your tank (like a 75Mhz Futaba 4-channel radio which Tamiya gave you a link for)? If two different radio rigs fail to run the tank then I'd think the DMD must have a problem. Your Spektrum radio is better than the Futaba they suggest, so I wouldn't be too quick to dump your Spektrum. I went through similar crap with my Tracker radio after I first got it (including shipping the whole thing to the manufacturer once, then the receiver a second time), but in the end got it working.
I'm sure DAK guys like Jeff, StreetSquid and EDoubleAZ Ethan may be able to help you figure this out since those guys all run Spektrums with their tanks. Like you said though, expensive is right, that is why I have yet to officially participate in this hobby by getting a tank of my own.
Good luck with the KV-2! If all else fails you can use a few Russian Anti-Tank Dogs instead..
