Yes, the sub level on the receiver is attenuation. All the way it will send the max voltage your headunit it is rated at. That doesn't clip. Your HU master volume does clip so just turn it up all the way until you hear distortion and then back off until it is gone. That is your max volume for actually listening.
If you wired the subs that way (+ to +, and - to -) and hooked them up on + and - of their own then yes you are pulling a 1ohm load because the speaker terminals on the amp are internally parrallel meaning its the same as connecting one sub to another then to an amp, just makes wiring easier. Any video on youtube on how to wire a 4ohm dual voice coil sub will clear it all out for you.
You don't need the remote bass control, I was just asking if you had it plugged in and could have been possibly turned down. Thats just for you to have near you in case you want to turn down your bass for whatever reason.
To tune your amp, do it by ear if you want a fast non-technical way of doing it. Find out what is your max volume level without distortion like I said and back off until it is gone. Write down the number. After that play at that level with a song you know well and all of its bass notes. Turn the gain until the bass starts to distort, sound bad, or not get any louder just muddier then back off a bit until it still sounds loud but clear.
If you want a little on the safer way of doing it, search up on google "setting gains with a DMM" (digital multi meter). If you don't have one you can pick one up at home depot for like 20 bucks. Some people say you are better off doing it by ear and others say the DMM method doesn't let you use the full potential of your amp. But unless you pay for someone to tune it with an oscilloscope or buy an SMD DD-1 it is your safest way to set your gains IMO.
This is what I can help you out at my best abilities. If anyone else wants to add to it or correct me, go right ahead.