the model number of the clarion sub is not specified. perhaps it's the old VNET PA-12S? nothing special about these subs; not very musical or accurate. i've modelled the specs before, and it would go better in a ported box of 2.0cuft, with a port tuning the box to ~35Hz. this may not mean much to you, but a specialist car audio installer could design and build such a box if you were willing to go down that route.
however, you can easily buy a new, better quality sub. subs that won't break the bank but are very decent would include an alpine type-S SWS-1222D, or JL audio 12W1v2, etc. such subs go nicely in a sealed box around 1.2cuft internal volume.
the pioneer amp is pretty decent. rated at 2x100WRMS, and bridging to 300WRMS with a 4ohm load. won't be as grunty as a good monobloc (1-channel) subamp, so will lack some accuracy at subbass frequencies.
i think if you want an upgrade, then change the sub first, and run it in a suitable subbox (specific to the sub). if your budget stretches, then a new monobloc subamp + sub/box would be nice. you still don't have to break the bank, with some very decent modestly priced gear around. for example:
*alpine MRP-M450 V-power series monobloc, 400WRMS @ 2ohm
*alpine SWS-1242D (dual voicecoil, dual 4ohm) type-S 12" sub
*1.2cuft sealed subbox
of course, there's a huge range of gear available, but to keep it simple, the above is what i often recommend cos alpine is fairly readily available, is reliable, and provides very good bang for your buck. i expect you'd need to spend around UK$200~300 for a new monobloc+sub+box combo of this kind of quality, or around UK$100~150 for sub/subbox. if you spend more, then you'll have higher quality.