3dB is not as bad as it might sound. You'll get 3dB of loss from 50ft of RG6. A fair number of diseqc switches hover around 3dB. My EMP Centauri has a heart-breaking 5dB insertion loss. If I remember correctly, barrel connectors can cost you around 0.6dB.
... that will not leak signal or pass voltage between SAT / TV outputs.
A few cursory thoughts on this.. We're in different bands, any regular TV service versus any LNB output, right? So signal leakage (or downright direct cross passage) should be of no consequence? Perhaps even, in order to keep losses to a minimum, it even seems like any intentional filtering here would be a bad idea.. I would further imagine the TV (antenna/cable) side is DC isolated with a transformer, as in a run-of-the-mill splitter. So passing any static voltage to that side isn't a concern. I've yet to open one of these up, so I don't know. I'm genuinely curious about the frequency separation aspect, however. I am going to try a few experiments tomorrow. Specifically, addressing this and glen4's comment on a good quality unit. I use two sets - one set is a supposedly good quality Philips brand that cost $15 each, in the insanely huge explosion-proof package. The other is a no-name $1.99 insta-rust set. Empirically speaking, I've noticed no electrical difference between them. I intend to quantify it, now.
Anyway, you do seem concerned about the loss. An amplifier will add a few dB of it's own noise, so if you're only compensating for the diplexer, it's likely a wash. As a practical matter, if you're on the edge for receiving your favorite channels, run separate coax.