Advice on new car speakers?

Here's the deal:
I have an old Ford Explorer.
Few years ago I replaced the stock stereo with a Pioneer Head Unit. I was pleased with the Pioneer because it made the factory speakers sound better

Now I have an urge to upgrade more.
I like my music loud with loud bass but after looking into it, I do not want to spend the $ for a new amp, subwoofer, + installation fee.

I heard that replacing stock speakers can make a dramatic difference, which I'm hoping to be true for these 13 year old Ford stock speakers-do you think any aftermarket speakers hooked up to my new unit will sound better?

My other question is, since I won't be installing a sub-are there any speaker brands known for having good bass. Or what specifications in a speaker signify good bass? My idea is to put 2 new speakers in front and 2 new in back but they would be two different types (one known for bass, other known for overall quality)

Yes I understand I will not have the bass I have always wanted w/o a subwoofer. But I don't think I'll have this vehicle for many more years so it's not worth it to me to spend a ton on the audio.

However, if anyone knows about good speakers for bass (and I'm also looking at these infinity ones for the front because they have good reviews on amazon), but if anyone has any advice I would appreciate it much. Thank you!

Note, all speakers need to be 6x8
And if someone could possibly explain to me how peak watts or how RMS watts work so I could shop for speakers better, Bonus point

Well, I do not know if you going to get a good bass out of a speaker but I'm pretty sure you will get a find good audiophile quality if you look into Sensitivity/dB. The more it is the better. Look for Sensitivity/dB that is 94 and up. Usually Infinity Kappa Series speakers come with it. I did some research and it said speakers with "midbass driver" might have some sort of bass with the sound. You might want to go to a local store before purchasing and testing it out. But for suggestion, check out this site for starters - http://www.onlinecarstereo.com/products/102/Car_Speakers/Midbass_Drivers.aspx

Example - Rockford Fosgate PPS4-6 - Sensitivity: 93dB -
http://www.onlinecarstereo.com/CarAudio/p_33433_Rockford_Fosgate_PPS4-6.aspx

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