No worries; just enjoy the ride. The more stuff you you experience and go through, the more you'll know and understand and it will come together. The trouble shooting and learning curve stuff is part of the journey and you'll appreciate things for what they are the more you learn. Don't be hard on yourself and don't take anything hard, this is fun, a hobby, treat it as such. Don't rush anything, it's not going anywhere. Take your time and just slowly learn stuff that interests you. Again, enjoy the ride.
So, just to back peddle a bit: your speakers are active monitors. What this means is that they have a built in amplifier in them and do not need a separate amplifier; they just need a line level (non-amplified) signal as a source. Your speakers also have an unbalanced RCA pre-amp line out for attaching a sub-woofer (top left on the back, called "sub out"). Your YU6's source input is a L & R channel RCA (unbalanced) and this is the most common to use so everything can work with it.
Keep your sub. It integrates into your speakers very nicely since they already have sub-out.
The only thing you actually need is a DAC/pre-amp with a phono pre-amp to accept input from your turntable. Your turn table's needle cartridge will take those vibrations and translate the canyons on your records into sine waves, basically, and the phono pre-amp will increase the signal and your speakers will then amplify that signal.
Here's a good, inexpensive phono pre-amp:
ART DJPREII.
That phono pre-amp will take the output from your turn table and output a signal that you then put into the back of your active (powered) monitors (YU6).
If you want to add another source, you need a different kind of pre-amp processor/receiver or a switch to simply switch between analog inputs (RCA connections likely, simple, unbalanced). But for now, if you're just turning records and listening to them, the above phono pre-amp is all that is needed to take signal from your turntable to your YU6 and your YU6 output signal to your sub. Simple pathway, no extra stuff needed.
Keep your sub. Return the receiver. Just get a phono pre-amp.
Very best,