Asking experienced .25 Marauder moders whats basic?

Personally on my .25 I only setup it up with the older SSG design and nothing else. It certainly helped flatted out the shot string and got me another magazine of shots out of it. That design has since been replaced by the TSS. Now if I had to do it all over again I'd pick up a TSS, and the new valve design Travis has worked on that I believe is being sold currently through a place he's partnered up with to do faster production.

Of course I'm not an expert, but I feel those are the two things other than a good regular that'd give someone the best results since they're drop in mods that don't require any permanent changes to the overall structure of the Marauder itself.

I have done alot of different mods, bought various drop in parts, and combinations of mods and aftermarket parts. A good valve like Travis has or Hill, is probably #1 for a .25. Past that it depends on what you want out of the gun. 

My top modifications in priority order:

1. Remove lawyer spring
2. Increase diameter of barrel shroud spacer holes to 3/16 inch
3. Pressure regulator (remove factory depinger & gauge block)
4. Increase transfer port inside diameter
5. Change hammer spring to one of the numerous "free flight" or "no bounce" designs
6. Aftermarket performance valve
7. MSD hammer
8. Shortened air tube
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   Modifications (1) & (2) are free and simple, so you might ask why is the regulator so high on the priority. All of my Marauders made an unpleasant ping sound when fired - the regulator eliminates this sound and of course the shot string is flattened. The transfer port change goes along with the regulator to aid in efficiency or it can be used to increase shot energy in an unregulated Marauder. "Free flight" or "no bounce" hammer springs improve air efficiency & can reduce report. Aftermarket valves improve power & efficiency. MSD hammers prevent air tube galling which then prevents cocking issues that occur on some Marauders. A shortened air tube improves the balance, handling and weight of IMO a heavy gun.
   
   Hope this helps.
   

Try here

https://docs.google.com/document/d/1Jn0RFfnH9YnFaEbT5ynizTnIyJ9DG6u5htJ_65R9P5g/edit?usp=sharing

On my .25 Marauder, all I did was replace the transfer port with a larger one (.162 as I recall), and install a TSS. I gained 100 FPS (25.4 grainers), without additional air use. 

I’m new to the Marauder myself. I have done a lot of online research to see what I can do to make my Marauder 25 better. The first thing I did was make sure the hammer was not binding.
https://youtu.be/CiXzq4WVA5s

next I added a regulator set at 150 bar I choose to use the Altaros regulator as it’s the only one that still allows the gauge to re ad tank pressure, I will say the Huma is a good choice also.if you swap the regulator oring from lower slot to upper the gauge will read regulated air pressure good for when you make changes.
https://youtu.be/K7NWbJOJZ-Y

after doing the above I added the marauder super hammer and the Mrod TSS available from jsairrifles.com I also have their replacement marauder balance valve ss on backorder.
https://youtu.be/AxvLgoM0bA0

after I have the above installed and my rifle is properly tuned If it doesn’t shoot well it will mean I need to practice more with it.
i found the following blogs to be most helpful in learning how to improve the marauder.

http://www.marauderairrifle.com/forum/viewforum.php?f=24&sid=270663d7ea4799fd4c6888be75310b2e

https://www.gatewaytoairguns.org/GTA/index.php?topic=130501.0

https://www.gatewaytoairguns.org/GTA/index.php?topic=131215.0