Had my Spark EV about a month now, joined the forums just to post on this thread
My experience with upgrading my speakers started with reading this exact thread and looking on Amazon at the Kickers that have been mentioned so many times. I thought to myself there must be a better bass-producing 4"/4x6 than those, and set out trying different models that had Prime shipping. Thus far I have tried numerous 4" as you can see below, here is what I found before
finding the EQ settings
EDITED 10/17, all reviews from before finding EQ settings removed, replaced with reviews now after having tested the speakers and
tweaking the EQ.
Rockford Fosgate Punch P142 ~$55 - (Would not fit without physical modification, did not test sound)
JBL GTO429 $70 - (Fit) Returned to Amazon before having found EQ settings and turned the bass up, can't give an honest review.
BOSS Audio NX424 $30 (Fit) Sounded good, I think probably the most bang for your buck if on a budget, sill not satisfying for me though, although bass was good upgrade over stock
Pioneer TS-A1072R $38.50 (Fit) Sounded maybe somewhere in between the BOSS and Alpine as their price suggests. These are newest I believe from Pioneer (in this class at least) and offer upgraded performance over TS-G series. Bass was about the same level as the BOSS but with slightly more clarity maybe. I would get the BOSS if you're on a seriously tight budget, or the Alpine if you are willing to invest an extra $15, for an extra $8 these don't make much difference
Alpine Type-S SPS-410
~$45 - (Fit) Sounded the best of all 4" speakers tested with EQ having been tweaked properly. These guys REALLY try when you have the bass pumped up in the EQ, before I found that and turned it up they sounded very meh, now they are kicking. You can get a farily serious amount of bass from these guys with proper tweaking of the EQ, and they will really start to jump before you hear any distortion. Highly reccomend if you are keeping stock speaker size.
Pioneer TS-G4645R ~$40 - (Barely fit, only able to get 2/4 screws in) Sounded overall slightly better than stock, slightly noticeable amount more bass
Pioneer TS-A4676R $42 (Same fit as TS-G) Sounded overall slightly better than TS-G series, bass able to be turned up a bit before distortion noticeable, clarity of sound otherwise also maybe just a bit better. For the negligible difference in price, definitely opt for these over the TS-G series. Just wish I found other competitors with 4x6 offerings to compare with.
After testing all of these and trying to get as much bass as I could out of the little guys, I was reasonably satisfied with the upgraded TS-A Pioneer's in the back and the Alpine SPS series in the front, with the money I had spent being under $100 after the other were returned. But was willing to shell out a little more for some more bass. I was reading a posting back from another user who had modified the speaker cutouts to house larger speakers, I believe he went for 5.25" in the front and 6x9 in the back if I recall. On to Amazon I went, bought myself a dremmel tool and the following:
Front: 5.25" Alpine SPS-510 $49: Night and day difference over the SPS-410, while only 1.25" larger in diameter the cone on them seems much bigger and when hooked up it shows
Back: 5x7" Alpine Alpine SPS-517 $57: Again night and day, easily 200% bass output over the Pioneer 4x6 and the cone looks like it has twice as much surface area too without doing some crazy math.
When properly tweaked, the new Alpine system will produce easily 200% the amount of bass as the upgraded
4"/4x6" system. I am still feeling it out a bit, but feel very satisfied that I no longer need a sub and amp. This again is using the stock radio and wiring, with some rather extensive tweaks to the built in EQ however. The upgraded system seems to need more help producing the bass it is capable of, I have the bass set to +10, mid to -1, and treble to -10. Modifying the front cutouts was a PAIN, took several hours dremmeling, and a lot of plastic shavings everywhere, still have not finished modifying the covers to fit properly without the speakers reverberating against them. Back 5x7 didn't require too much modification, but did drill some screws through the plastic mount, so it most definitely is not for those on a lease.