It’s is as good of an idea as it was a month ago.
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buying: I prefer Thomann. Their extended warranty is really nice and their service is one of the best I have ever experienced (it’s worth for me not going for the cheapest).
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keeping the DT: It’s a good idea to keep the DT around for a while. You may like to use both (for example by offloading drum duties to the Digitakt) and the OT may require some time to get into.
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MK3: definitely not likely. Maybe a black case version in the future, but that doesn’t change anything in functionality. And “no”, to me the OT doesn’t feel old. It’s much more capable than the Digitakt (the Digitakt feels too much restricted to me).