Emotiva XPA5 Gen 1

My same comments on Emotiva XPA-5 also apply to A-300.  If you really want a brand new amp with that much power, Emotiva is hard to beat.  But you can easily get a used amp that is significantly better!

Emotiva Preamp - same thing.  They are rather lackluster in the midst of other preamps, but if you really want a NEW product, it's hard to beat. 

Preamps are actually a lot harder to buy cheaply than amps.  You have to spend a good amount of money before you are able to get something decent.  If you are looking for sound quality, this Vincent is probably the minimum I would recommend (it's all manual source selection and volume - no remote):

http://tmraudio.com/components/preamplifiers/vincent-sa-31-stereo-line-preamplfier-sa31/

The Vincent is actually a quite excellent performer.  It get's hard to find a good preamp without spending $600-700 or more.  If you want bargain basement, look at Schiit Saga. You can then upgrade the tube to something like Electro-Harmonix 6SN7 to get better resolution.

http://www.schiit.com/products/saga

If you're willing to spend more, Schiit Freya for $699 is outstanding.

If you want to stay on the solid-state side, Parasound P5:

https://www.usaudiomart.com/details/649439594-selling-parasound-p5/

Hope this helps.

For your subwoofer, you will most likely have to get a couple RCA splitter cables like the following:

https://www.amazon.com/AudioQuest-splitter-female-15-24cm-cable/dp/B000GUSQJG/ref=sr_1_2?ie=UTF8&qid=1528944801&sr=8-2&keywords=audioquest+splitter

You would connect the splitter cables to the right/left RCA outputs on your preamp.  The splitters allow you to further connect separate left/right RCA cables to the amp (for main speakers) and to the subwoofer inputs.

Sometimes you can get an amp that has 2 sets of RCA outputs.  The Vincent has two sets.   On your Onkyo preamp, you can use the single RCA subwoofer output on the back/top for the Klipsch subwoofer.  Just connect it to either the left or right input on the sub.

NOTE: Even though the Schiit preamps have 2 sets of outputs, you can only use one at a time.

The Parasound P5 actually has an analog crossover built in so that you can split the frequencies between MAIN RCA outputs as separate SUB RCA outputs.