I need some advice and opinion, please.

I have decided to run my TV sound through my KEF speakers via a Pioneer
amplifier. I've used standard RCA phono cables from the Line Out (L &
R) on the TV into the (now redundant) tapedeck inputs on the amp. Note
that there is no AUX input on the amplifier.

The sound quality is poor - muffled and muted, compared with say, the
sound quality from the CD or FM radio. What am I doing wrong, anybody?