That box looks fine and the price is in the ballpark, but for guitar amp work neither 1% precision nor 1 Ohm steps are necessary. Also the housing seems to be plastic. It will be noisy if you run signal through it. Another choice: http://www.parts-express.com/pe/showdetl.cfm?Partnumber=320-076&FTR=RESISTOR%20SUBSTITUTION
I bought some old Eico & RCA cap/resistor sub boxes on eBay. I really like them; though some values have drifted. And they're Bakelite with aluminum front panels. So no shielding either unless I add shielding paint or tape internally. They seem to go for anywhere from $10 - $45 so its a crap shoot, but yo can win at a low price eventually.
To run signal outside the amp chassis under test, I use shielded 2-wire mic cable.