What kind of bass do you want? Do you want deep pounding bass? Just a better sense of front stage bass but not necessarily extremely deep? If you try for 6x9s with no amp and crank it constantly there's a good chance all you'll end up with is a very nasty 'popping' sound every time you hit a heavy bass note.
I think you may be like me. Im not an expert but I got into this 2-3 years ago and am still learning alot. With what I spent trying to get what I want to hear on the cheap and then gradually ugrading to where I am now I could have some of the higher end non-chain-store brand names on everything in my truck right now.. Again, I am not under any circumstances an expert but if there's anything I've learned is that 'cheap' is not inexpensive. Trying to do it on minimal money with minimalist equipment won't work if your expectations are unrealistic.
You will probably be disappointed because when you say you want 70$ 6x9s that play 'bass' alot of people think you mean 'midbass' and are coming up with suggestions based on this assumption. Youll buy them and wonder why you don't get big pounding boom in the front stage like you were expecting. I think alot of times here some people say 'I want bass from the front speakers' and the more knowledgable people take it for granted that it's already understood that strong 'midbass' is the best you can expect unless you're going for a VERY elaborate front stage. So you will be disappointed with the lack of boom and will probably be back to ask what's the cheapest good subwoofer.
70$ coaxials plus the 60 dollar subwoofer you'll end up buying next=130$.
You will probably blow the sub and want a better one. When you get the better one and proper amplification for it you'll notice it now drowns out the 70$ front coaxials you have, at which point you'll probably be back asking what are the best front comps with a 150$ budget. 70$ coaxials + 60$sub+100 dollar sub+150$ set of comps to replace the coaxials=380$ or so.
But wait... you'll need new amplification for both the components and the sub. So take about 380$, add another 200$ retail for a decent sub amp and 2-300$ retail for a decent speaker amp, you're now at 7-800 dollars.
So the real choice, if you're trying to get subwoofer-type bass out of 70$ front speakers, is, do you to spend your 7-800 bucks now and be happy for the next few years or do you want to spend it piecemeal and finally be happy 3 years from now after having your car ripped apart 3-5 times?
Cliff notes: Rather than trying to get bass from front speakers I'd go with AT LEAST 1 really good 8 in a small box somewhere and then a decent set of front coaxial, if not 150-200 dollar front components.