how to upgrade this digital front end ?

Less expensive suggestions are welcome, but it must be a vast improvement......

accuphase 75v, used or discounted, plus a few dozen redbook cd's. one box, no patchwork clock correction. run it direct to your amp if you wish. vast improvement, fur sure. -cfb

Pick up a used PS Audio P300 ($750ish). Keep the ATT/ST and see how it sounds without the digital lens between the transport and dac.

looking for vast improvements on redbook cd is an exercise in futility. you will find subtle improvements, if you listen super long and hard.
as cfb pointed out however, the ergonomic improvement gained by going to a one-box player may be considered 'vast'.

I agree with Cornfedboy. I'm using an Accuphase DP75 and I love its musical almost analog sound! The DP 75v is the latest version which also plays SACD's and has an even better decoder section. However, if you have your doubts about the future of the SACD format (as do I) and are content with the conventional CD redbook format, then you can save a bunch of cash by buying the 75 instead of the 75v if you can find one! And with the money that you save, you can buy a great turntable rig and PLENTY of audiophile vinyl and CD's!