Modulated Channel Interference

I recently bought a 3-channel modulator and installed it on my home cable
system.  I found out that I was getting serious noise due to not having a
low pass filter on the line so I purchased on of those.  Still not real
happy with the results.  Now my channels 2-6, 16-21 are fuzzy and probably a
few others.  The modulated channel isn't even that clear.  If I just hook
the modulator into the combiner the modulated channel is very clear. I
thought purchasing the low pass filter would solve this but it seems to have
created a few other problems.  I'm thinking of scrapping everything and
returning, getting expensive.  Any suggestions before I do?

Thanks

Barclay