Amp rear speakers or buy subwoofer?

[Blu 420Sel]

I just installed a Sony CDX-730 in my 1988 420sel. I've left the stock spearkers in the dash and front doors for the moment and replaced the rear speakers with 6.5" Infiniti Kappas. The look and performance of the head unit is terrific and the clarity of the sound has really improved. The only downside is that the lower end is pretty weak. I'd like to add a little more bass to the system without blowing lots of money. My options are:

1. try to find a discrete subwoofer to mount under the rear shelf (but I don't know which one). I don't want to fill up my trunk with lots of equiptment though, I want something small and inconspicuous if possible.

2. amplify the rear speakers, but they're only 6.5" speakers, I don't know how much of an improvement it would make.

3. reuse my old amps. I think they actually worked in spite of their reputation. My system actually had more bass before the swap (though the clairty and head unit performance was pretty bad). Still, they're 15 years old and I doubt they'd really compare with a contemporary amp.

Any thoughts would be appreciated, thanks!