I sort of disagree with having to add an amp for your existing speakers to regain the same amount of bass you've had previously. If your bass is really that bad, then I'd go as far as to say that maybe they forgot to connect the rear speakers to your head unit.

The factory speakers are probably not made to handle very much power, so by adding an amp to them, you're probably guiding yourself to a bigger mess.
Installers from BestBuy range pretty widely. Sometimes it's just some high school kid, sometimes it can be a very experienced installer who just lost his previous job in this economy, but I'd say the overall jurisdictional for BestBuy is that it's not a very good place to go to if you want some quality product / work done. If you have any bit of electrical or engineering sense, the install for a head unit is really not extremely difficult, and members here can definitely help you out step by step. There's always the option of speaking with Broadfield (or forum car audio seller:
here or
here). I've purchased from him a couple times and he's been awesome with his service. He's even gone as far as to say that you can call him on the phone for help during an installation if you have issues or problems.
I currently have a single 12" Hertz subwoofer and it's given me plenty of bass, and I'd say it's more than enough for a lot of people. A well done 10" install would probably give you enough bass for your personal taste by the sounds of your post. Going with two 12" subwoofers is pretty overkill in my opinion. Regarding the branding issue, I'd say different brand makes different good products in different areas. For example, I love what Kenwood has done for their head units, but I love Audison for their amps. There's no need to stick with one specific brand for everything.
For the position you're in. I would do the following:
1. Check to make sure sound is coming out of every speaker.
2. Read this guide
here and go through the previous install job to double check everything and to learn about your car.
3. Once you've checked hardware, I'd look for a "factory reset" for your Kenwood head unit to reset your software setup.
4. If you still have problems, check for some car audio specific stores and bring your car in for some troubleshooting help. (Looking on forums for shop recommendation is greatly recommended)
5. Once you've solved all your current issues, re-evaluate if you still want to add an amp and a sub.
6. Talk to Broadfield about what you want, and your budget, do some online shopping to compare prices, and get them in your car!