Reddit mentions of JBL GTO939 GTO Series 6x9" 300W 3 Way Black Car Coaxial Audio Speakers Stereo

These are my recommendations. Are your 6x9's old? Because, they are cheap speakers, but they still shouldn't distort like that.
Some good 6x9
http://www.amazon.com/gp/aw/d/B003KN2TGM/ref=mp_s_a_1_6?qid=1398354095&sr=8-6&pi=AC_SX110_SY165_QL70

Some good 6.5" rounds
http://www.amazon.com/gp/aw/d/B003KN2TF8/ref=mp_s_a_1_4?qid=1398354234&sr=8-4&pi=AC_SX110_SY165_QL70
Finally you said you wanted to maybe add a second 15"? This amp has 2 channels, so you could add the second some at any time. As for knowing if it is 2 or 4 ohms, you can use a digital multi meter (dmm) to measure the impedance in the sub. This is a 2 ohm amp, but a 4 ohm isn't hard to find.
http://www.amazon.com/gp/aw/d/B003CYL7LW/ref=mw_dp_mdsc?dsc=1&qid=1398354444&sr=8-12
I hope this helps!

I had this driving my two of these and the amp was just entirely too much for them at anything past 15 volume on a scale to 50. It definitely is decent, but I don't have the means to test its actual output.

Coaxial speakers just means that the tweeter and woofer and built together, so you only have to install one thing. Components come with an external crossover(sends bass+mid to woofer, high end to tweeters) that everything connects to, which lets you place the separated tweeters in a separate location.

Dunno if your particular focus has seperate tweeters, or coaxial speakers, see if this video is your car. If it is then the size looks like 6"x9", which would be great if you like bass because 6x9 speakers are the next best thing to a dedicated sub-woofer.

Don't see any well-known brands of 6x9 speakers on Halfords, but Amazon.co.uk has some nice looking choices. Like these JBLS, if they will fit.

You'll have to take time to do a little research, to know what the exact size is. Or rip off the door panel and break-out the tape-measure.

Yes sir, Ram 1500.
I was looking at these for the 5.25" and these for the 6x9. I basically want a tweeter up top since the speakers are so low in the door.

http://www.onlinecarstereo.com/CarAudio/p_46055_JBL_GX962.aspx?utm_source=google_shopping&utm_medium=Product_Feed&utm_campaign=google_shopping_Price_Comparison&gclid=EAIaIQobChMIl-e13MeT3gIVT1qGCh0gdAIxEAQYCSABEgL0QPD_BwE

Above is gx series. A little over 2 ohms but 94 dB so should be ok.

JBL GTO939 GTO Series 6x9" 300W 3 Way Black Car Coaxial Audio Speakers Stereo https://www.amazon.com/dp/B003KN2TGM/ref=cm_sw_r_cp_api_mYLYBbDP4CDHQ

3 way gto series which is similar to what I had, I had the 2 ways as I felt that 3way can just be marketing talk at this level. There may be better deals out there.

I can't recall but I may have replaced the dash speakers with gtos as well, been a couple years.

I like This MB Quart amp.

and if you don't go with Focal take a look at these Infinity Kappa 6x9s or These JBL 6x9s