I would say an SDR receiver is much better at most things then something like an R390a. The Flex 6300 is a poor example, its well down the list and well below the old Flex 5000! A good SDR will have the noise level of the actual receiver much lower then the R390a, and lots more ability to set filters where you want. The idea that you will not be able to repair a modern radio is somewhat true, while you CAN work on them and repair them, some parts may not be made in 10 years and out of stock in 20. But its much more likely anything that runs on a computer will be useless as computers will look much different in even 10 years. Another advantage of modern gear over old gear is its much more stable. Buy a new SDR and chances are you could use it for 10 or 20 years without having to do anything to it, as many have with the old Flex 1000. The old R390 and DX100 will break a gear clamp, blow its mod iron, melt a 1625, blow its ballast tube, have that cap protecting the filters short out, and mechanical filters and PTO's do wear out! So you are going to have to fix the old stuff, maybe a lot. I like both old tube gear and modern gear, one is not better then the other overall, just different. At the moment, the best performing receiver you can buy is an sdr.