I'm quite happy with REW covering most of the extra things a digital scope does,I think Im gonna settle for an older tektronix analog scope as soon as I find one.
I did check out the AD2 ,
It does seem to deliver the goods in terms of processing power , but the hardware is a bit cheap and flimsy looking , fair enough you are getting access to the software which probably accounts for the lions share of the price . But as previous contributors have mentioned the typical RTA/spectrum software packages are hard to live without nowadays ,even if you do happen to have a super duper scope ,your still going to want something like REW on hand .
The point was raised about Analog discovery 2 not being able to interface with REW , there really isnt any satisfactory USB audio signal probe on the market , I'm having reasonable success with a modified centrance axeport as a front end to REW , but thats long since discontinued , and there isnt much in the way of inline usb interfaces that can do proper 24/96 , one that seems to fit the bill could be Pyle's Microphone to usb adapter , a humble enough 39.99 US$, might be worth a look but asio drivers are unknown and input would need modification to get to 1 meg . I'll probably keep an eye out for an original mic port pro just as a quick and easy balanced input to usb for test purposes.