friend's 94 Viper, wants more bass

Alright I have that just bought a new viper..well its not new its a 94 with only 28k on it and he got for a heck of a deal.  Anyway they put an after market system with Alpine 6.5s in back and 6.5 components in the door.. They also put in a JL 300.4 for the amp.  Getting it to "sound right" is a pain because they installed the tweeters near the handle facing directly at you and the 6.5 is down at the bottom of the door and it gets completely blocked by the leg...and whatever is coming from the back...

Anyway, here is my question...he was wanting to increase his bass but doesnt need a whole lot of boom..So I told him an 8" or a shallow mount 10" since space is limited but he wanted to know if the 6.5s in the back could be swapped out for ones that had more bass, I said yes and no...So after all that, I need suggestions for either a sub or 6.5s that would fit the bill...Im thinking maybe Adire?

And if he went with the sub, I guess the first option due to space would be bridging that 3rd and 4th ch on the amp and run the sub on that..what do you guys think..

Thanks and I appreciate all that I have learned from you guy

If space is limited you could use one of Boston's 10" G2 subs. They can fit in a small .5 cubic foot box and sound pretty good. https://www.crutchfield.com/S-4L90dZGnHAk/cgi-bin/ProdView.asp?g=67600&I=065G21044

Image dynamics make subs that work well in very small boxes also. The 10" IDQ would probobly work well receiving 150 watts continuous from the bridged channels of the JL amp. https://www.cardomain.com/item/IDSIDQ10D2V2 You might want to put it in a ported box though, because you only have 150 watts to work with. This would be a good SQ system but it probobly wouldn't get too loud.