I lean towards not making drastic changes in a system unless you 1. get a ridiculously low price (one you can't walk away from) on something you had no intention of purchasing 2). know for sure that that piece of equipment is the problem you are trying to fix. So, you have older Adcom amp and pre-amp and an older CD player. again, I don't know the age or quality of your DAC. But, I wouldn't buy anything without first auditioning it in my system. So, reading what you originally posted and the speakers and amp/pre-amp you have. I would audition a good quality external DAC first. do your research, set a price point for each piece of equipment and go find some to listen to in your system before buying. There are some really great DACs out there and some high quality used DACs also. Same for amps and pre-amps. For example, a Theta DS Pro Gen VA DAC. You would be hard pressed to find a better DAC new now and used you could get one for not a stupid amount of money. The only issue with DACs as with other digital equipment is they always are adding new bells and whistles. Such as HTMI for DVD players and USB inputs for DACs. If you don't need USB input for your DAC at this time, then you really would have a hard time finding a better DAC new or used. The computer audio crowd wants DACs with USB so older DACs that don't have it are on the used list. My standards are quality music and elimination of listener's fatique. If something in my system is making me get up and not listen to music for long, then I will find out what it is and replace it over time. Remember, this is a long term hobby, not short term. Don't do everything quickly or at once. take your time, one piece at a time. Based on what I have read about you, I would do DAC first, Pre-amp second, Amp third and maybe CD player last. But, each time, do your research, set a price point and budget, go borrow or audition in your home that piece of equipment and enjoy the journey.
enjoy