I can get hold of some Brand New BB OPAMPS from USA. The seller is quite reliable.
I bought OPA637BP, OPA2107AP and OPA2228BP from him. My experience is changing the 2604 to any one of the above does give a lot of difference.
The 637BP is single channel, we need TWO instead of ONE.
The price should be around US$ 10 - $ 13 including shipping; depending on the quantity.
I got quotation (stock available) here in HK for 2107AP at HK$ 135 @
OPA 637AP is at $ 187 @. All are Brand New from a reliable source.
I am currently using the BB OPA2107AP. The sound is more open and clarith is very high compared with the original 2604. Will try the 637BP after I get hold of the PCB board.