Question about length of SPDIF COAX interface cable length

That's surprising.  Brightness would suggest a presence of the jitter (that translates to added noise).  What about clarity/transparency and imaging?  It is remotely possible that some electrical noise is injected from the source, possibly thru the shield, while longer cable, having more shield inductance, impedes electrical noise injection more, but I'm grasping at straws.  Did you keep both gear pieces in the same physical location for both tests?  Did you notice it at the same time of the day?  Often people experiment after work and dinner around 6-7PM, when radio stations power is suddenly reduced by FCC regulations (better signal propagation at night).

I tried it with .5M, 1.0M, 1.5M cables on my own DAC and prefer the 1.5M as well..

I would also pick 1.5m from these choices.  8" (0.2m) shouldn't have reflections at all (not a transmission line).

grm,  I assumed typical 25ns transition time, but perhaps it is different.  Maybe your source is swinging faster, especially at smaller capacitive load (shorter cable).   Whatever it is - ears are the best judge.  Same goes for different 1.5m cables - cheap cable can have by chance better impedance matching with your gear, than very expensive one, resulting in better sound.  

Hi kijanki, thanks for your kind assistance. In another room of my home where my Home Theater setup is located, I have connected the output from my DAC in my main system to the HT system DAC input. Using some return air ducts I got lucky and was able to install a 9M COAX cable I made myself. The cable is a 75ohm quad-shield terminated with 75ohm RCA style connectors. Anyway, my point is that this connection sounds great even at 9M (I think the max you can go is 10M) so something is going on with cable length with relationship to sound quality. Question is, is there a formula you can go by to determine optimal length of COAX cable? For example eliminating a transmission line by keeping a cable under 12", or in multiples of 1.5M which is my case here? I do believe not having a transmission line to be the best place to start but it did not work out for me with respect to SQ.

grm,  I'm not sure if there is any formula, and case of your 8" cable shows it.  What counts into calculation of transmission line effects is transition time that can vary between 5ns for dedicated transports to over 50ns for average gear.  In addition the highest slew rate is usually at the beginning of transition (first knee).  When calculating trace length on PCB it is a little easier, because logic family is known, PCB material is known, etc.
At the end, result is what counts and your ears are the best judge.