Actually, the Hijack method is not bad. I was just listening really closely on the new install (XM Sportscaster) with the AUX boosted 35 points (17.5db?) and it seems very clean.
The other install was a Roady XT. Although the Sportscaster is supposed to be the low-end unit (I think it looks sleek), the Roady XT install has a lot of noise that seems to be getting amplified by Hijack. I initially thought it was a problem with the method Hijack was using to boost the volume. I guess there is a ground problem or something similar with the Roady XT install.
(This still doesn't help XM's declining compression quality
)
EDIT: Edit to reflect Hijacks boost number compared to dbs