Looks like that is a HTIB type reciever that comes with speakers and a sub.
Confirming what you already know, crossover appears to not be adjustable, no known easy way to just send the full signal to L and R speakers.
https://www.avsforum.com/forum/90-r...13-sony-str-k7000-ht-7000dh-speaker-size.html
the subwoofer output is a standard run of the mill unpowered LFE feed (it's meant to plug into a powered-sub)
https://www.docs.sony.com/release/STRK7000.pdf
If you wanted to merge the LFE signal with the speaker level outputs, you would need to amplify the LFE signal and then find some way to merge it with the stereo channels ... It would probably be expensive.
I don't know how to do this, but I'm sure others do.
Maybe an alternative option would be to replace the reciever with something different/better?
Since full feature Recievers are gonna be like $200+, would maybe a T-amp work well for you? for around $20-30?
for one of my PCs and my Wife's PC's we use decentish speakers (my BIC, her Dayton Audio, note: not as good as 12 inch Vegas!)
We have our PCs connected to T amps,
Something like this little Lepai
https://www.amazon.com/Lepai-LP-202...ocphy=9021479&hvtargid=pla-569395004374&psc=1
It's not a full feature reciever, so it does't handle switching across multiple input streams and HDMI and whatever, but, for just pure audio, they work magnificent. Also, the sound goes reasonably loud and has full range.