30 year old player, worth an upgrade?

I added a $130 Topping E30 DAC to my 30 year old Denon CD player, and it made a notable improvement.

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.