You ought to live with the thing for a while before you have it upgraded. Otherwise, you'll never know if the upgrade was any good.
Your desire to not use a preamp is important. I'm not sure how the SB does volume control, but it's probably not an ideal solution. What I would do is a get a DAC with a good volume control solution and use the SB digital out into it, and don't bother modifying the SB, at least not initially. For digital out, the SB is really quite good as is. Benchmark and Lavry are good DAC suggestions (I own both), but neither will give you remote control over the volume if that's something you need.
Your desire to not use a preamp is important. I'm not sure how the SB does volume control, but it's probably not an ideal solution. What I would do is a get a DAC with a good volume control solution and use the SB digital out into it, and don't bother modifying the SB, at least not initially. For digital out, the SB is really quite good as is. Benchmark and Lavry are good DAC suggestions (I own both), but neither will give you remote control over the volume if that's something you need.