I'm not going to post a counter claim that MOTU A/D integrated mic pres are simply great, 'best thing since sliced bread'. They're not. But in my experience they are functional. I have not used any variation on the MKIII but started with 828 with original (before there were any 'MKs' at all). I've had a MKII that I've used, primarily for live and/or field recording, w/o issue for about seven years. The units were (& I'm assuming are) built to price. But the primary issue, as always, is budget. How much can you spend, how much will you have to spend to see appreciable improvement (not merely, for example, improved work flow (another vital area for gear purchase)) in Audio.
The expensive part of boutique mic pres remains in the analog circuitry (sometimes the logo). I can buy the IC OpAmps used in one of my best, most flexible pres for roughly $1.87 ea. (I don't replace every outdated IC chip I own with those because not every OpAmp performs well with every topology . . . but even when I've removed egregious components, recapped, etc improvement from replacing 1987 OpAmps with genuinely better 2010 ones is marginal, at best)
If the issues you have with MOTU are corrected to your satisfaction then you don't need, initially, to be overly concerned with replacing the pres. Make some music figure out the limitations of room & gear chain. Get a better feel for what improvements will return most significant results. (If one is doing any ambient recording (i.e. voice/instrument in a room) then anything you do to improve the ambient environment pays off handsomely. If you're doing recording designed to be listened to anywhere other then headphones then anything you do to improve ambient mixing environment returns significant benefit. For example). It would not be surprising to, in a relatively short time, actually need some better mic pres. But there's still a fairly wide gap between ones that will 'get job done' & ones you'll keep even when their technology is old & decrepit (V72 for example). Price point on a pre one does not more or less have to guess to be significant improvement stubbornly remains, roughly, $500. Have not checked prices recently (but there are certainly a lot of threads on the subject) but ones to audition are: FMR RNP, Black Lion 312 & Auteur, True Systems P-Solo, Audient Mico, DAV BG1. Never used the Golden Age but gut response would place it in the pack near the FMR, but undoubtedly worth checking out. While mic pres are far less important (to final sound) then either room or mic not every model of every manufacture flatters every voice. And the final caveat is that I don't, as a rule use the pres on the 828, in fact I typically avoid using mic pres integrated with A/D convertors. But I can't think of a scenario where I'd shut down a session if I need two additional pres and all I had left, available were those on the MOTU.