this little guy for $14 shipped is passive, line level, and has adjustable cutoff. If it does what it's supposed to, it seems perfect. The reviews on this other listing make it sound like maybe it doesn't really filter correctly. Or maybe it's just first order (6db/octave) which doesn't cut as much of the highs as people tend to think. Anyway I'd love to know if it works, if you want to try it ;)

Otherwise, I second AzusMobo's recommendation to pick a cutoff frequency and buy FMOD inline rca filters.

Lastly, if you know your amps input impedance, you can make your own line level filters, which should be cheaper; but I don't know how common it is to know that specification. (In fact, I'm not sure how the FMOD ones are getting around that issue. Maybe all modern amps are similar enough in that regard?)