I am the last person who should start this thread but I thought it would be useful to pull together some comments from other threads. (Stan, Mods and anyone else-please correct anything I have got wrong)
Stan has provided a very high quality dac which is designed for easy modding especially with regard to the op-amps which are socketed. The stock NE5532P works fine but to put it in context are 62 pence each at Digi-key.
The replacement op-amps that I have seen mentioned to date are:
LM 4562 (recommended by Stan)
OPA2132
OPA2134
THS4032
LME4970HA (circular unit, might involve cutting legs)

The 4562 costs less than £10 for a pair from ebay. Trailer has noticed an improvement with this one as have I-fitted it today.
So lets see some more suggestions and feedback. Dont forget to detail cost and where the parts are available from.