impedance question

The speaker level inputs on your sub should have very high input impedance and will not draw power from the amp OR effect it's ability to drive low impedance loads.
You could, if you wanted to run the mains full range, just wire them as normal and a 2nd set of speaker wire...and it could even be magnet wire, to the inputs on the sub.
You'd know if you were loading your amp to 2 ohms or less. It'd get real hot, pretty quickly and/or simply would not be able to play very loudly.

Speakers ONLY draw what power they need, to the electrical limits of the amp. Power 'ratings' on speakers? Sort of a weird science thing. Better watts....are better.

Adjust the phase knob after getting the main speakers running and at maximum perceived bass. Just swap +- lead on ONE speaker and if bass disappears / diminishes put it back and forget it. Than add the sub to the equation, adjusting the phase knob until you get max bass again. It's lowest level should be 180 degrees from the maximum level.

If you are dealing with a HT receiver, you are doing something wrong in the 'setup' menu. Set speakers to 'large' or 'small'.....that sort of thing. I'm not an HT guy.

I once upgraded from a 200wpc amp to a 100wpc amp. The lower powered amp had much better bass. I have two pairs of speakers both rated 86db/6ohm. My 12wpc t-amp makes nice music with one, sounds shut in and anemic with the other. My buddy's Bedini 25/25 sounds awesome with Totem Ones, a speaker that can make many more powerful amps (wpc) sound constrained.
There is so much more going on beyond wpc and impedance ratings. They can only give you a limited idea about how an amp will power a speaker.

Read your manual. If you don't have one, do a Google search, I'm sure you can it somewhere. Good luck.

I don't know why my sub output on my pre amp doesn't work. I just tried again and get nothing. Next question is there's a switch on the sub, since I'm feeding it from my mains do i select the LF filter "active" or "off"?

Hi it is a self powered sub. In fact 1500 watts!

If the sub is in line with the speakers set at zero. If some other position experiment with the phase setting. Why don't you feed your sub from a line output from your pre to the high level or line inputs on your sub? It almost sounds like you do not have a self powered sub. Is this so?

Since I got your attention, what is the phase knob for on my sub and where should it be set at?

Keep in mind that the 4 ohm speakers are most likely less efficient than the 6 ohms speakers (ie 85db vs 87db) so the increase in wattage output won't be as drastic as you think.

So can I attach my sub to my mains via a parallel connection safely?

Yes, given that it is a powered sub (i.e., it has its own built-in amplifier, and an AC power cord that plugs into a wall outlet), and given that the sub has provisions to accept speaker-level inputs from the outputs of the main power amplifier.

Regards,
-- Al

So can I attach my sub to my mains via a parallel connection safely?

Also, the impedance rating of a speaker is its nominal rating; most speakers will have an impedance function, and associated phase angle (a function of the driver/cab design and crossover type), that vary widely across the operational bandwidth. And, most listening is performed at levels with the amp cranking out, again nominally, 1 watt or less.

1)Those power ratings are the MAXIMUM power capability that the amp is rated to CONTINUOUSLY deliver into the stated impedances. The actual power delivered is determined by the volume of the music at any instant of time, and will generally be far lower than those numbers.

2)No, that is not correct, assuming that the sub contains its own amplifier (as is usually the case). The input impedance of the amplifier within the sub will normally be vastly higher than the impedance of the main speakers, and therefore will present a negligible load to the main amplifier.

Regards,
-- Al