Speaking of the spackle issue - last night I got my Omnimic measuring kit from Parts Express and wanted to measure my speakers. I moved my left and right speakers around a bit during testing and put a huge dent in the spackle on one of them
Sooooo ugly - gloss black with a BIG white divot in it
Luckily it's on the back, so not really visible in general. Just kills me to know it's there. The rest of the speaker is veneer.
Regarding the costs ramping up with trial-and-error - i spent 10 times as long as probably 5 times as much money working on the finish to my speakers as i did on building the MDF cabinets themselves
I tried the veneer route and there is A LOT to learn in that realm and each step takes time, money and patience. But it does look beautiful if you can get it right.
Vinyl or leather can look nice too - i had good success with vinyl on car audio boxes back in the day.
Take your time since you'll have to look at this thing every day (unless you're completely hiding it). I'm psyched my speakers match my furniture fairly well, so it was worth the time and effort.
Good luck. Definitely post some pics of your final cabinet.