30 year old player, worth an upgrade?

It certainly is. Digital sound quality has come a long way in the last 30 years. If you are going the DAC/Transport route, try to find a DAC that is less than 5 years old IRT technology. 

Thank-you for all the responses. At the moment, I have no intention of going separates but all 3 players under consideration have digital outputs so running them as transports + DAC in the future is possible.
In Malaysia where I am from, used hifi market is not as big and dynamic as that in the States, so a used Apollo for example will not be an easy find.
The price of the players I listed is equivalent to around USD650 for the NAD and Bada. The Marantz CD6006 would be 40% lower given that it is an used item.
So I guess my budget would be around USD 650~ 700.

with the CD6007 released, prices are good on the CD6006 and it should way outperform your CD40...I always found the expensive Sony DVD players to make lousy CD transports...

Good to hear that the modern Marantz should be better. This will make me less hesitant to upgrade.

Although I own a Marantz hdcd-1, I also have an Audiolab cdt6000 transport. I use a Border Patrol Dac. Good combo.

Hello,
I would make sure it has digital outs. If it has RCA/Coax even better. I like the idea of you adding a DAC later on. R2R like the Denifrips Ares II is better on SS gear. If you are running tube gear a chip based system would’ve better. Like the Topping DAC.  Apollo has reliability issues. The NAD is built like a tank and sounds like a traditional CD player. The BADA is nice but I don’t know about reliability. Marantz/ Denon product sound a little warm. If you are running tube gear get the NAD. If you are running SS get the Marantz. This way you will balance the sound a little. Get a 14- 12 gauge power cable for the CD player. It will add weight to the sound even if it is a cheaper Amazon or Chinese cable. If you can swing the $240 go with the Puritan Classic Plus power cable. It mellows out that digital sound and adds sound stage and decay to the sound. Or should I say it lets the music play as it should.  I also like to use Puritan power cables on streamers  like a Sonos or BlueSound and DACs. Good luck and I hope this helped.