If you feel you would benefit from more power then using your 600i as a pre amp with a W-8 amp would be a step up. You might get a little more control and punch in the bass with the additional power.
You could also try adding a P-7 or P-8 pre amp and using your 600i only as a power amplifier. I was surprised to learn how important the pre amp is in the overall sound - maybe more important than the amp so long as the amp is powerful enough for your speakers.
A third option is adding a W-7 and bi-amping your speakers, with the W-7 driving the bass of both speakers and the 600i driving the mids and highs. I tried this with the I-7 and W-8 but the gain on the I-7 was higher than the W-8 so it didn't work (the higher gain of the I-7 made the mids and highs a little louder than the bass).. It's worth a shot with the 600i though...many swear by bi-amping as it creates a truly effortless sound and really can open up the soundstage when you offload the task of driving the bass to a separate, dedicated low frequency amp.
Definitely try these options before you buy, though, as the benefits over the 600i alone may be relatively small for the expense. The 600i is an impressive piece! Also, Simaudio integrated amps have the advantage of really short, high quality connections between pre amp and amp without the need for expensive interconnects.