There are plenty of amps that have a speaker-wire input. But that would mean losing the use of your rear speakers (unless you amp them too).
You can get a decent CD player for £50 on most sites. Mine Sony was £95, it has CD-MP3/USB/AUX and outputs for fronts+rear amps and a dedicated sub output. This lets me set a low-pass frequency from the HU and I can adjust the volume of the sub too. I think cheaper Sonys (£60ish) will do this too. Not sure on other makes but I'm sure they have something similar.
If it were me, I'd get a new HU and amp a sub via RCA outputs, its the best way.
To hook it up. Drill a hole through the passenger footwell for the amp cable and run the amp cable from the battery (make sure there is a fuse on the cable in the part of it under the bonnet), through the hole, along the passenger side. My plastic trimmings where the carpet meets the doors sills unclipped and I managed to easily squeeze the cable in. Run this to the boot. Along the drivers side you need to run an RCA cable and amp remote wire along the drivers side to the boot from your HU. In the boot wire up your amp and earth it to the chassis - normally to a seatbelt bolt or something that will secure it to the chassis. Get some decent speaker cable and connect from your amp to your sub and nearly done. Next secure both amp and sub securely in the boot. Screws, heavy duty velcro or boot install - your choice.
I've been really happy with muy JBL amp and sub, but am thinking I'd prefer a 12" ported or 15" rather than my 12" sealed as the bass sounds too tight.
Hope this helps.