Subwoofer Out VS Subwoofer Crossover

Your post omits a lot of relevant information.  The Yamaha Sub output frequency and slope is not specified in their spec sheet or in the manual, although some online say it is 100Hz, others say 90Hz.  [In the past (before digital bass management) Yamaha always had the sub out at 140Hz, 12dB/Oct].  You neglect to identify your subwoofer, except to say it has speaker wire in and out.  Some subs put an electrolytic cap in series with the speaker output affording 6dB/Oct. High Pass attenuation to the attached speakers, others don't (and whether an audiophile wants a cheap electrolytic in series is also a question).  You also don't say what your speakers are, and whether your system's subwoofer implementation up to now has been satisfactory to your taste.  Your new amp has no preamp in/out connections, so a real crossover cannot be used.  My advice is probably to continue using it as you have been, but filling in the missing details might make me revise that opinion.

I disagree with some.

1.  Bass is the bigger power user from your amp. I would plan on 200W unless subs are very efficient.  Powered subs.are fine.

Splitting the signal is fine.  I don't run full range through my mains.  IM distortion is in mains when you use them with out a sub.  Making your mains sound better with better bass is the goal.

I have an external crossover.  I adjust gain.  Crossover set at 100.  A good crossover is $600 on up.  I have Marchand and I just read about another one, can't remember the name. 

Two subs are better than one.  Mine are streo but can be summed to mono.  Multiple ones are easier to integrate.  I feel my subs and mains become invisible. 

Isolate your subs from the floor.  I used springs but there are many options.  

Thank you all for chipping in. I never realized how little I know now that I’ve acquired this integrated amp that is a bit more "stout" then anything I’ve ever owned. So my first question was yes, a little lite on info. This stuff is probably beginner level stuff. I’ve always wanted to own a system with a good solid punch. So, I found myself grabbing the yamaha integrated amp, 2 polk speakers, and a powered sub.

The amp is a Yamaha A-S701 (manual). It maintains a single RCA out for the sub. It also maintains an impedance selector.

I’m running 2 Polk Monitor XT70s (manual).

I have a 100W Sony sub with speaker in and outs, as well as a single RCA in, and an adjustable cross over dial.

My thoughts were to remove 80 or so and down from the speakers, and allowing my sub to take care of 80 and under, aprox. My understanding is that this keeps the amp from having to pull a lot of low freq weight. As well as free up the speakers for a bit more volume headroom, in order to play loudly, safely.

I’m also new to the impedance setting. The speakers specs/manual shows they run at 4ohm. I’m not perfectly sure if I’m to set to high or low on the impedance filter. I'll be simply playing 2 speakers out of A.

Thank you again. I’m excited at what appears to be a nice leap up in home audio experience. 2.0 that is.

@whitneyhengesbach

Points to consider... Your amp is very capable of powering the mid-base drivers of the main speakers no problem. 220 watts into 4 ohm. Plenty loud.

The Polk XT70’s are capable of going down to 35hz/40hz. Why buy speakers that can go this low if your not going to let them? 2-61/2" mid base drivers with 2 8" passives and then cut them off at 80hz and then hand it off to ONE weak 100 watt subwoofer?? Why do that? Huge mistake IMO. The XT70’s are very capable of adding LOTS of punch and dynamics far better than one, weak 100 watt subwoofer can.

An upgrade you might consider is splurge on a pair of better powered subs like SVS or REL and set them up in stereo and then cross them at 40hz to blend properly with your speakers.

Or...

If you don’t add new subs I’d probably set this system up using the speakers full range and leave that sub out altogether. I bet you’ll be very impressed with the bass extension of the XT70’s all on their own. 35hz IS VERY DEEP BASS! No sub needed. Especially not that sub. No offense but you’ll probably make it sound far worse by adding that sub.

Good luck!

Given the new information I agree with @gdaddy1. I would set the system up without the subwoofer running the speakers full range.

If you use the system for video also you could try the subwoofer via the RCA input when doing so. Value of this approach would depend on how low one can set the subwoofer crossover.  Set the crossover to lowest setting. You would likely find yourself sub turning the subwoofer off for music, maybe not using it at all..