I', sorry that I can't give you specific advice on the Fubar IV, but as a very content owner of the Fubar III for the last three years or so I'll share some thoughts that hopefully might help on your quest.
I've owned both the Fubar III and Headroom's "Micro Stack" (micro DAC + micro amp) - the micro stack has tons of accolades and was about 2.5x the cost of the Fubar III, but after months of close listening I could in no way say that the micro stack sounded 'better'. Point of the story is that my Fubar III sounds great and was relatively inexpensive.
Can't speak to how the Fubar IV sounds, either by itself or compared to the III, however I would recommend the III as it is cheaper and will sound, if this is even possible, more 'natural' than an upsampling DAC. Upsampling is at best controversial technology, and many many many educated audio persons see it as nothing more than a marketing tool without any possible benefits. If you're interested in spending money on upsampling tech, I suggest doing a little research and finding some objective sources on the matter.
As far as the aftermarket PSU and op-amp rolling for the Fubar IV, I would recommend against both of them as well. I have both the extra PSU and have rolled my opamp to the OPA627, which is one of the most expensive, "golden-standard" op-amps in the audio world. Neither of these extras makes what I'm listening to sound any better. Actually, neither one makes what I'm listening to sounds any different, either. And of course at the time of buying them, I was totally convinced they would be making a positive impact (duh, why else would I spend the money), so this is not a biased case of self-confirming pessimism.
What you are going to get in spade here on head-fi are tons and tons of people crawling out of the wood work saying things like:
'ZOMG, i increased the humidity in my room by 3.7% and now my phones sounds AMAZING; the highs now produce an iridescent glow and the micro-clarity has extended in length but not so much in height, which is just what I wanted!!!'
ie, it's a place where you're going to hear a lot of very dramatic, totally exaggerated opinions and ideas on any/everything. However, this is not how I see the world, and especially the hi-fi audio world, working. Unless you're talking about different headphones or incredibly cheap or faulty equipment, the differences between what a human can actually distinguish is not like how it's made out here on head-fi, where even the most inconsequential of details produce the most incredible of affects on the listeners.
Exagerated opinions of sounds and exagerated opinions on brands, too. Unfortuntely for Firestone company, if they were ever a 'flavor of the week' I dunno, but they certainly aren't popular here anymore, and you'll find a lot of people recommending against them, probably without ever hearing them for themselves. I've heard my fair share of amps, dacs, amp/dacs, some cheap and some expensive, and for the cost, I see the Fubar III as an amazing product.
Of course I'm not talking about anyone who reads this, just about some of the other head-fi members of course *winking smilely face*.
My real recommendation though will be to google the O2 amp and buy that. The O2 dac just came out and you will be able to but the combo amp/dac shortly. The O2 combo I think will be selling for slightly less than the Fubar IV and is in a whole other league of awesome-sounding awesome-soundness. They are being sold out of JDS labs. The "O" stands for "objective" and what you will be buying is a combo amp/dac with proven ie OBJECTIVE capabilities.
Sorry about your wallet,
Edited by oaklandrkg - 5/8/12 at 6:50pm