Those who mentioned the resale value of an upgraded amp being less than a similarly priced new amp are correct. Therefore, you need to decide if you plan to keep and listen to the amp because you like how it sounds, or if you are more likely to get on the buy/sell merry-go-round. If you like the amp, and just want to make it even better, then the SMc upgrades are a great value. If your goal is a better resale margin, then the upgrades are not for you.
I wouldn't worry about the age of that amp....it is a very good design and built like a rock. It will be dead nuts quiet and totally reliable when Steve is done with it. As I recall from owning the DNA-500, and using it with a wide variety of tubed preamps and mostly with a Tom Evans Vibe/Pulse SS preamp, the amp displayed very good clarity, dynamic, hard-hitting bass, and good dimensionality. The weak point was a small bit of wispiness in the treble, which Steve says is corrected as part of the upgrades. I do not remember resolution and detail being issues with that amp. Maybe you need to try a SS preamp like the Tom Evans or better yet, Steve's own TLC-1 with Signature upgrades, which is the best preamp I have owned out of about 20, including several that were north of $10K.