What makes you think the DAC is making your speakers sound a little bit bad? Does music sound better with an analog source than a digital source? Can you quantify the sound quality you feel is missing when using the integrated DAC in you receiver?
There's a lot of hype around DACs, and it may be true, but I remain a skeptic for the most part. If you're really concerned about the DAC you'd probably be better off using an external DAC instead of buying a new receiver, the HK is arguably a better amp than a Yamaha.