speakers and cables

 I ended up with this, it has more (too much) bass for sure but I will play with it, check the phases as advised above and figure out the right settings. It did push me to dislike my amp as it has connection issues, I almost have to hit it with a hammer when I turn it on.  

Yes, that is what I suggested, it should work just fine, as you want all the power to do to the mains first, then fed the subs. 

Now comes the "hard" part, setting up the subs. Would start with both sub gains in the middle, and crossover at the main speaker low cutoff. You don't want both main and sub to be pumping out the same frequency, you want the subs just below the mains to fill, not add to. If you think it's too much, most likely they are in phase. Out of phase it can sound hollow.

On the volume, listen to a good bass song, and try to dial in the sub volume to match the mains. It will MUCH lower then you think. My subs on my HT are set to around 3 o-clock and 60hz XO. 

@immathewj yes you are right. An integrated would do it. If ebay didn't charge an arm and a leg, I would sell my power amp today. 

@mswale my subs' dial start at 80Hz so I can't go below that. I agree with you that less is more, I need just subtle bass to extend the image. First it felt like the drummers went mad :)

@grisly, what are the specs on your mains?  Or did you already say how low do they theoretically go, and I missed it?

"@immathewj yes you are right. An integrated would do it. If ebay didn’t charge an arm and a leg, I would sell my power amp today. "

There are many sites that are better than ebay to buy and sell audio gear including the site that you are now using! 

@yogiboy

Nobody buys what I have on any site other than on ebay. I know, my gear has been posted for close to nothing for 8 months. On ebay I get an offer in 2 days.

Because the stuff is heavy, and ebay charges on the shipping too, I get 60 cents on the dollar. In this case, I would make around $80 if I am lucky.