For me, how much is enough is when I say it's enough. That declaration has nothing to do with what anyone else is doing. No one else's spending habits effect me in the slightest.
My upgrades have been influenced by what I heard in my audio friends' systems, which showed me what is possible in audio. But I always thought on my own terms about the amount of money that I would want to spend.
I agree that enough is enough when l say it's enough. At this point I have such high performance and value price ratio that I really don't know how to efficiently upgrade from here. I probably have one of the best monitors you can buy (for system, see signature), extremely resolving and musical digital source and amp that provide a first class signal to the speakers, probably some of the best subs you can buy for a medium sized room like mine, and some of the best cables for a still reasonable price (I am not willing to spend 5 x more for a 10 % increase in performance).
Sure, the monitor/subwoofer concept itself is a compromise. But so is every system concept that I have heard. And the positives of my current system are much greater than I had hoped for (I am thrilled, frankly!), while the disadvantages are much less than I had anticipated.
So is it enough? At this point I'd say yes, for the reasons cited. Might I suprise myself in the future? Sure. But until my current system, I had a clear and reasonable upgrade path. Not anymore.
Sure, there are better DACs, but this DAC is so incredibly good that I would have to think in terms of 20 x its price for a potentially really substantial upgrade (except for the current version 2 upgrade for this DAC itself). Would I want to spend that money on a top level DAC, an item that in terms of price/performance ratio usually becomes obsolete quite quickly? Not really.