what to upgrade?

Hi all, new poster here but have lurked on/off over time. Anyway, I have a compact pick-up that I'm currently running infinity Refference 5010cs components (75w) in front and Polk DB 650's (60W )in the rear on a Sony XM-4 (50WRMSx4) amp. Sound is vg good for my tastes but I have an itch to scratch and a few hundred $ to spare.  I was thinking of a) adding some depth by adding a Pioneer 8"shallow subwoofer  to the system. ( It HAS to go under the front seat and I don't have room/want to loose rear seat storage. It's about all I can find that I could make fit) . To do this, I was thinking that instead of matching a 120-150W amp for the sub, I'd be better off buying a 75-80W 2 channel amp for the front components and bridge the freed channels on my existing amp for the sub.  My questions are, a) is it worth the rewire effort to gain the 30% extra watts for the components or should I just add a matched amp/sub set independantly?  Also, I know this is a small sub  and that it will be limited in it's contribution so would I be better off just spending the $ on upgrading my door speakers/amp ?

All thoughts/advice appreciated.

p.s. I'm an old fart  and am listening to blues, country, etc. No metal, hip hop, etc. 

Well any true enthusiast's answer to the question "what to upgrade," no matter who asks it, is always "upgrade everything you can afford to upgrade" of course. 

If it was my system the first thing I'd look to upgrade is the head unit (you didn't mention what it is?) and the front speakers.  After that, adding a sub will make a huge impact on any system, then look to improve the quality of your amplifiers.  And have fun! 

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Thanks for the reply. Sorry. The HU is a JVC KW-XG700. I like it for it's features, ease of use and looks. ( Not to flashy.)

So would you replace the Infinity Ref (front) door speakers before adding the small SUb? Just trying to see where i'm going to get bang for the buck.

I'm pretty happy with the infinity's. Especially the tweets. The mids are pretty clean but only 5.25". In hind sight, I wish I had stuffed a 6.5"set  in.  ( and wondering what the next step up in speakers would be/sound like?)

I'm going to do the Sub I guess.  Leaning towards rewireing the fronts to a new amp  and using the existing amp for the sub as well.  

Thanks for your time and  input. It's appreciated. 

I agree that you will appreciate the addition of a subwoofer now.  Two questions are lurking:  one, about amplifers.  And two, about the front soundstage.

For amplification, I would go with a mono amp dedicated for sub use.  Class D and most efficient use of auto's power supply.  Problem is finding one that is that small.  Try to find one that puts out that amount into 4 ohms (could be 200 watts, really) and use a single coil 4 ohm sub.  Use your existing 4 channel amp as long as it performs to your expectations, whether changing front speakers or not.

If a wholesale amplifier change is in your way of thinking and you have someone to give your current amp to, a 5 channel amp would be a good prospect for your system.  The fifth channel is that dedicated sub amp I referred to.

6.5 mids up front, assuming door damping is done as well, is better in reproducing those critical freqs in the 80 to 120 Hz range which is where you are crossing over to sub freqs.  Better than 5.25s can do, that is to say.  Tweak the system as you have the fronts now and if you pinpoint a lack of snappy drum response and some dimished bass guitar notes, look at upgrading the fronts at that time.  Make a note of the higher range and tweeter response you are satisfied with now and listen for the same type of sound when you audition new components.

Thanks again for the input.  I'm going to take your advice and go with a seperatly amp & sub ( in a custom enclosure. Now I just need to figure out which Sub/ enclosure I can fit. ( An 8 is easy, a 10 is pushing it but might be doable ). I'll make a foam mock-up of a maximum sized box and then figure out how much volume I can squeeze out before fabrication.

Beyond that I see Blaupunkt makes a class D amp that will give me 190W RMS. I'm figureing that will do nicely for the 10" but  I'm a little bit concerned about running a 125W rated 8"  on it.. I'd have to watch the gain and HU output etc I guess.  I also see Kicker has a nice 5 channel w class D sub section that would fit the need if I decide to just change out the existing amp. (which sure makes wireing easy.)

As for the doors, I'll watch for a set of  6.5" components/ mid woofers to swap out the 5.25" eventually.