I have the Fubar III too, but also along with the dedicated PSU. Not sure which version you have, but mine is the 2009, which has some minor upgrades from the previous model.
Recently, I bought a pair of HD-650s. They sounded great with the Fubar III & PSU, but I wanted the HD-650s matched with something a bit more reputable. I upgraded to HeadRoom's Micro Stack (Micro amp & DAC), which cost about double the Fubar III/PSU.
After several intensive listening sessions, I was really unimpressed with the Micro Stack. Given they cost twice as much as the Fubar III/PSU, I expected a lot more. Actually, I have some questions regarding this and might start my own thread to ask them, which may be useful for you as well. If I do make that thread, I'll PM you the link if you want.
Back to your question, I had a long talk with Firestone's North American distributors about the Fubar III vs. the Fubar IV, among some other things. Basically, the amp part is unchanged - the chief differences between the two is the 24bit/upsampling DAC, and the added optical & coaxial inputs.
Unless you really want those added inputs, I feel that the IV is a waste of money. The increased sound quality produced by the IV's upgraded DAC is questionable at best, and most likely just sounds different but not better than the III. 24bit is hotly debated among the audiophile greats, so for causal users such as ourselves, the benefits (if they actually do exist) would truly be pearls before swine.
Instead of throwing $350 right out the window, my suggestion to you is three-fold: 1) upgrade the op-amp in your Fubar III to the Burr Brown OPA2627 ($51 + shipping), which should produce a noticeable improvement, 2) purchase the dedicated power supply unit (PSU) ($117 + shipping), which I guarantee makes a noticeable improvement, or 3) do both, i.e. upgrade to the Burr Brown OPA2627 op-amp AND get the PSU. Going with option 3 will cost $168 and produce a significant gain in quality, while your original idea of simply replacing the III with the IV probably won't produce any objective gains and costs nearly $200 more than what I've suggested ($168 vs $350).
Funny enough, this was how I felt before I had talked with someone who was part of the Firestone 'family', and I was rather shocked to find that the person I talked to was in agreement with me. Given that, and on top of my personal experience with the Fubar III, I feel pretty confident in my suggestion(s) to kick the sound quality up a notch or two without breaking the bank by just throwing the money out the window. Even if you were setting out to purchase a Fubar for the first time, I think you would get a lot more out of the III with the PSU ($348 for the combo) over the IV ($347). That extra dollar for the III & PSU will carry your tunes further than the IV will.
The only objective gain as I see in the IV is the optical & coaxial inputs, but depending on your input needs, that could be a rather moot selling point.
After listening, back to back, and comparing the Fubar III/PSU with the HeadRoom Micro Stack (new @ $666) , I have to say, I think the Fubar's value is quite exceptional. I am by no means a certified hardcore audiophile, but my ears are sharp, and with all the tummy-rubbing that HeadRoom receives, I expected a lot more for doubling the price of my old system.
AlwaysAfan: unless you are willing to play the wallet-choking, diminishing-returns games of a true audiophile, I think that the Fubar III is an exceptional choice for value and quality. There is really zero need to upgrade to the IV (inputs aside), and it's not even imperative you follow my other suggestions either, I just threw them out there to give some effective/cost-effective ways of improving on what you've already got. For someone not interested in continually breaking their bank in search for that 'better sound,' the Fubar III is a rock-solid unit.
