I've had my K701s since 2009 and they see daily usage, haven't had anything happen to the headband.

I've been using a Little Dot DAC_I and Little Dot MKIV SE setup for my AKG K701s for quite a few years now and I've been pretty happy, sadly the DAC has gone to meet its maker and instead of spending a lot of money to ship it to/from China for repairs I figured I'd get something new instead...

I got a whole bunch of driver tubes already, but somehow I found myself enjoying the Mullard EF92s the most on the MK III at least. Still need to get a pair of EF91s though.

I just got my MK IV SE to replace my MK III, and I have to say... wow, what an improvement. Using the stock tubes atm, but I'll soon migrate my 6N30P-DR + Mullard EF92 combo from the MK III. The MK III made my K701s sound great, the MK IV makes them sound totally awesome! Obligatory pic...

Seems to be the price for 1.

Take a peek through the holes at the bottom of the unit, you should be able to make out the revision of the pcb through there. Can't remember exactly which hole you see it through though.

I went with eBay, and it really depends on your wallet. 6H30P-DR tubes can be a bit pricey ($100-$200 for a matched pair, depending on age).

I can agree with drews, I'm using Mullard m8161s and 6H30P-DR power tubes, and I'm also very happy with them. Can be a bit pricey though if you go for the DR variant.