Whilst what you say is logical - personally Id still go with the WA12. The guy who designed it really knows his stuff. Whilst Nebula is an awesome ITB tool - applying Nebula to a clean track - and recording with a nice Pre - are different things...thats what Ive been lead to understand anyways...
I've decided to come clean..I mean go clean! I was tempted by the Grace but decided to go F*******e ISA One Digital. I must admit I've been been a bit put off the Grace by all the sterile claims. Especially as 'they' say it's better for instrument than vocals. Actually I think it's all a bit splitting hairs so I'm basically going for the cheaper option! The money I save will ease the pain of a Benchmark DAC1 as well. And it' not like the F*******e is a compromise.
For anyone who's interested I've spent some time researching DACs and it seems that there are some very good options in the hi fi/audiophile market. They're considerably cheaper too. Have a look around - you'll be surprised at the choice. I'm going to audition the Cambridge Audio DACMagic plus - it's £350! Highly regarded on Gearslutz by people who know more than me. The Cambridge seems a good choice in that it has Asio drivers, asynchronous usb, headphone outs (for 0 latency hardware monitoring) and XLR analogue outs.
I'm no expert in converters - I reached the mid range and have owned the likes of of RME, Lynx, Apogee, Sound Devices and so forth. So I don't know how much better Lavry, Weiss, Prism etc might be. What I do understand however, is that you're not going to hear their benefits without even more expensive monitors and room treatment. The Cambridge will be just fine for mixing if it matches the RME level stuff!
I went through a similar process in wanting to upgrade my DAC, but since I had the luxury of being able to get three models on loan for audition purposes as well as a Lynx Audio soundcard, and then write up my findings for Sound On Sound magazine, you may be interested in the results (especially since my 3 choices were an Apogee MiniDAC, Benchmark DAC1, and the Lavry DA10 )
I think I remember that article when you wrote it...
Moving on up in gear is a game of diminishing returns as you say, and I'm really quite tempted just to forget it all and get an RME ADI-2. I can after all, happily mix on one and the masters seem to come back well. In the context of this forum, any Nebula library makes much more difference than conversion at this level.
I owe it to myself to audition the big boys when I go down to buy the RME or Cambridge Audio. It's the only way to really know. But I do wonder if they will impress me enough to make it hard to pull the trigger on the RME?