After you've saved up, I suggest your next purchase(s) be upgraded tubes for the iDSD Pro and/or iPower Elite 15V power supplies for the Pro stack.
Upgraded tubes will increase the distinctions between solid-state, Tube, and Tube+ modes and make the sound more three-dimensional and holographic.
The iPower Elite power supplies will eliminate any background noise from the stack and/or the tubes (you'll only notice the noise after it's gone) and take you to the next level. Get one for the iCan Pro and one for the iDSD Pro, and you can roll whatever power cables you'd like with them. I find iFi's SupaNova cables (I know, I know, another purchase) to have really good synergy with the iPower Elites and the Pro devices, but any power cable you have on hand will do.
I've fully upgraded my Pro stack, and I love the improvements to the sound.