Using a sub with no sub out on my amp

I just checked, the 12" has no Port (my preference and advice). The 10" and 8" models have a rear port. If not near corners ok, but avoid them if possible to achieve the kind of imaging the pair is intended to achieve. Single sub, a port can get 'more' but I find it's risky, you can get 'more' mud.

Luckily, it’s not the amp, it is the speakers that are lacking in bass. Any self-powered sub with speaker wires in/speaker wires out will solve your problem.

I just added this 800 wpc self-powered sub to my small office system.

https://www.rockvilleaudio.com/rock-shaker-12-wood/

On sale, in your budget, 90 day trial with free return shipping (I bet you keep it like I did).

It gives a nice fullness to my bookshelf speakers. Don’t over-do it, just a bit, un-noticeable until you turn it off.

Your speakers are 89 db sensitivity (better than my Wharfedale Diamond 225 87 db)
Your amp is 30 wpc, more than my Luxman’s 10 wpc.

Your woofers are 5-1/8" diameter, they can be great but can only move so much air. Mine are 6-1/2", same thing.

When your sub’s amp takes the low bass job away, the bookshelf speakers then do a better job with upper bass/mids/highs and thus imaging improves along with detail ... no distortion from trying to do what they cannot do.

You will be happier at lower volumes after you add a sub to those speakers I bet.

Prior response, add single sub, I assumed a space without a lot of space like my small office.

IF you have the space for placement, and listening position to enjoy imaging, bass and the overtones of the bass notes give directionality and stereo imaging.

You might want to try a stereo pair of subs. These are ideal for that, front firing, no port. Place l/r adjacent to your mains.

Easy to try, Rockville will let you return the second one if you find it’s not worthwhile in your space.

If you go for a pair, perhaps their 10", or even their 8" will do, scroll down on the same page as above

https://www.rockvilleaudio.com/rock-shaker-12-wood/

you know your space and budget. You can give them some info and ask their advice.

Strange that the amps sound SO different in the same space and same speakers. Is there an impedance mismatch somehow, phase reversal, .... ?

Speakers on your desk. IF you spend a good deal of time in your office, have you considered changing everything in there? I’m retired a few years already, finally weeded unneeded stuff from when I ran my business from here, and accumulated stuff. That opened up possibilities.

Our rooms are similar, mines essentially 11’ x 11’ room, difference is I pulled my desk back away from the front wall (wharfedales ear level inside bookcase opposite), got myself/monitor/keyboard dead center. Whoopee.

Cutting the bookcases down to 3 shelves high and putting a slightly larger top on them made a huge difference.

You can see my office system (or anyone’s system) like this:

Click on their user name. Click on Details. If a system is shown, click on it.