Can a DAC sort out a flat\cold amp?

The amp itself can’t be the problem unless it’s defective. Yamaha integrateds always have strong bass, no reason an amp wouldn’t. Here is a review that mentions the bass being too strong: (skip to "The Listening" 2nd paragraph)
https://www.stereonet.com/uk/reviews/yamaha-a-s1200-integrated-amplifier-review

Nor should a soundcard be the problem; any soundcard from 25 years ago can play music fine. Except in the case that its software or some other software on your PC has auto calibrated for the amplifier in some gross way.

Check your wiring to speakers and amp, your software, and your amplifier’s manual.

edit -- if you really need to confirm your room isn’t the problem is there another space you could take the amp and speakers briefly? Also, as somebody already said, try with just a CD player, game console, etc bypassing the PC. -- sorry for redundancies, your last post came in while I was writing this.

+1 @crustycoot.   Some audiophile don't want to accept that flat frequency response and low distortion can only have one result as long as the output resistance is not high.

The absolute cheapest way I know to solve lacking bass and too much highs is to attenuate the highs.   Solder the capacitor onto an rca and plug it into the Tape Out jack.  Usually, the tape out is connected before the volume control.  Right in the signal path.  Value? Depends on the frequency the top end starts getting hot. .47mfd ? to start?  This is where Radio Shack excelled. 

Too bad the amp sucks to you this way.  And now having to resort to putting the band aid on your elbow when your knee is bleeding.

OP, thanks for your reply. Honestly, If you are going to move in six months… I would not make any additional purchases. I would wait.

If you were going to remain there I would recommend downsizing your speakers. But moving is going to completely change everything.

I would like to reinforce how incredibly important your listening space is. I lived in small houses for many years… then bought a house with a large incredibly asymmetrical room I am now using for my audio room. Holy cow… biggest upgrade ever!

So, you want to improve your system in the future. Look for a place you can set up a 6 - 9’ triangle for speakers and chair… one where there is nothing in between the speakers and the speakers are away from the front, back and side walls. Think about thick wall hangings for dampening (see my systems under user ID).

if you anticipate rooms similar to what you have now. Then consider getting some small stand mounted speakers like Totem (with a small subwoofer)… natural sounding and punchy… less room dependent than than too large floor standers in a tiny room. Right sizing is key.