I told many times about A1-A2 differences. A1 can 5.1 sound and has SNR=100db. A2 SB0240/0280 can 6.1 sound and SNR=106db. A2ZS SB0350/0360 can 7.1 sound and SNR=108db. Both A2 generations can play DVD Audio 96 kHz (P16V unit of EMU10k3 DSP).
Then, about 5'25 Extensions: all Live/A1 units are interchangeable, i.e. you can use any Live 5'25 Extension with any A1 card and use any A1 5'25 Extension with any Live card which has 40-pin header. The A2 5'25 Extension can not be used with Live/A1 cards because it has another pinout.
But if you want to use A2 for sure plus full functionality in pure DOS you may continue DOS mixer project for A2.