Rear Speakers for 89 SN

Hi - I’m restoring an 89 SN which appears to have had 4x10 rear speakers mounted in the rear seat base. Not sure if this was stock? Looks like 4x10 marine speakers are hard to come by. Any suggestions on a better solution for rear speakers (where to mount, size, etc). Front speakers appear to be 6.5” which are easily replaced so no issues there. Also, no tower on the boat so tower speakers are not an option. Would appreciate any tips to consider in a new sound system. Looking for quality, clear sound with a good bass that won’t break the bank. Plan to stream everything from my phone so a head unit isn’t needed. Thanks in advance!

Since you specify you'd like to have a decent amount of bass, steer well clear of the 4x10's.   I agree with Ross in that I've never seen a marine version in that size anyway, but if you did find some the bass reflex from that platform isn't very good.   I don't know what you'd do with the original openings, maybe just leave the speakers in there and don't hook them up? Or you could go to the trouble of patching the holes and recovering.
You can find a few 6.5" coaxials that provide a decent amount of bass, but you won't be able to just get anything, even if it's a top brand. What you need to look for is low frequency response at a higher efficiency.   Speakers rated at 40Hz or less for the low end will produce better bass, 50Hz and higher will start to sound a bit tinny ... especially in an open air environment like a boat. The higher efficiency rating the better as well - 89db is kind of low and to me, not acceptable whereas 90-91 is good (average for the better speakers) and 92+ would be very good. The higher the efficiency the less power it takes to produce the same sound volume.
If you prefer more bass, upgrading to a larger speaker like the 7"-8" round will usually get better numbers in the bass department. The drawback is that with a larger speaker area you'l generally be needing more amp power to achieve the same volume. Also, they are always more expensive and of course you'd have to modify your openings. Thus, if you're not adding an amp, I'd stick with the 6.5" platform.   

Just because it has a big name doesn't mean you will get big (bass) sound - look at the specs first. Reading reviews can help but IMO they aren't 100% reliable for personal taste information like ' do they provide a lot of bass sound?" - One guy's 'awesome' bass is another guy's 'Where's the bass??' so take those with a grain of salt.

Great suggestions. Thanks for the tips!!! Thinking about going w/ a amp, 4 - 6.5'" speakers and a sub-woofer to go in the storage area under the bow. Anyone else have a similar set-up?