amp playing music not just bass

You know by now how to set the gains, and if you're not sure, you know how to find the info in these archived threads.  So, take care of the gain first.

Use the LPF for the subs.  Some amps have steeper crossover slopes that minimize the amount of higher freq music that you can hear from the subs, and some amps have less sophisticated filters that, when set, will still allow a fair amount of "music" coming from the subs.  This is in the design and build of the amplifier.  It will not hurt your subs to play these higher freqs;  the only problem, possibly, is that it will adversely affect the soundstage from your listening position.

If you are relying entirely on subs to make a sound system, this poorly built LPF could be a nuisance.  But if you have amplified front speakers in the car as well, the little bit of highs coming from the subs shouldn't be too much of a detriment to the overall sound.

BTW, the choice to make in upgrades?  Don't add a crossover or other device in the signal path.  Instead, upgrade the amplifier.

If you have activated the LPF and set it to 100Hz or less and you still "hear music" through your subs, the LPF in the amp is shot .  Legacy is a marginal quality company so I am not surprised.  If the LPF is out, the rest of the amp may soon follow.