Aged Adcom Conundrum/Question

At present, all of my listening is done via headphones (Sennheiser HD 580, Creek OBH-11 and Adcom GCD-700). To my my ears the sound is excellent, being free from excessive grain and harshness, yet possessing clarity and detail. I've seen Adcom GDA-600's for sale recently for prices below $200.00. Would it be an example superfluous redundancy to add the GDA-600 to the GCD-700? Logic and reason say no to me. But foresight and praticality say yes. The aged GCD-700 is at the end of it's expected lifespan. With the GDA-600 on call, a cheap player/transport can be purchased with the promise of quality sound due to the presence of the GDA-600. Though obviously having no mechanical moving parts, the GCD-600 is just as old. Should I consider a Beresford, Zero DAC, etc? What would be your advice?
I bought a new Adcom GDA600 when they came out. i bought an Adcom GDA700 used, several years later.
I still use the GDA700 as my primary DAC.
I recently (one year ago) tried a Stereophile class "A" rated DAC ($2,200.) and returned that DAC because it was no better than my old Adcom GDA700 DAC. (I added a used 'VAC Sandard' tubed preamp used by me as only a 'fancy tube buffer' to ameliorate the HF digititus of the Adcom GDA700 which was apparent throught my Bryston/Magnepan setup after i returned the unnamed new DAC.)
Also: When I bought the GDA600, I bought a Rotel transport with it. When I later bought a End of the production run local dealer Demo Sony SCD777ES, I tried it with the Adcom GDA700 DAC. The Sony as transport via the GDA700 TROUNCED the Rotel via the GDA700, using the same CD, connector...etc .. (I was embarassed to sell that Rotel it was so bad compared to the Sony as transport)
THe Sony SCD777ES via it's own internal DAC sounded about equal to the Sony SCD777ES-as-transport via GDA700 DAC.
So that Adcom GDA700 ia a good DAC. NOT a world beater, but pretty good.
The moral of that comparo is a cheap transport is NOT going to make the GDA700 sound good. A decent transport is needed. (Nearly any used ES Sony, or better Yamaha, or Pioneer Elite player is good, I use five disc carousels like the SCD333ES and SCD555ES, CD9ES, Yamaha 7XX series..)
Finally, i personally think the Adcom GDA700 can be tweaked a bit: I add anistatic foam in baggies filling the interior of the chassis up. And a good powerline conditioner is also very helpful. Like I said the Adcom is the equal of a not to be named by me again (i called it out when I had and returned it) Stereophile Class "A" rated recent reviewed DAC that at $2,200 was only as good but NOT better than my Adcom GDA700.

Now a few other folks have condemned the Adcom DAC as suck-city in various posts here or in other sites, So your mileage may vary. My take is the tweak, and a good powerline conditioner make the GDA700 competitive with current DACs at twice the original price, or ten times the Adcom's used price. I still have kept my GDA600 around, just because, along with the GDA700, which is not leaving my system for a long time more.
(I also own a TADAC DAC I bought for it's headphone use)
i own Magnepan 3.6's Bryston BP-26 and Bryston BP 1.5 and Bryston 4B-SST2 amp, and use the Adcom GDA700 with them.
So I have a lot of experience with the Adcom GDA700.
The GDA600 is nearly as good as the GDA700. It merely lacks the balanced out, and HDCD decoding. With the foam tweak the GCD600 is as good as the GCD700 (except for decoding HDCD. The internal structure of the two Adcom DAC is identical, as i own both, and have had both open at the same time. The 700 has just few bits added in)
If you can find an Adcom DAC, it is worth using. whether it is better than the internal DAC of your Adcom CD player, I do not know.. but used players are nice when you have a DAC you can depend on. i own four changers i can use via my Adcom DAC. Ps i would spend only $200/$225 for the 600, and $250/$300 for the 700 at this time. IMO.
And they never need any repairs... very reliable.