Yeah, maybe that's my issue.
It's the specific combination of Sound card (Audigy 2), settings available for the card (hardware tone/treble, EAX EQ, etc), my speakers (Z5400) and my own presets. I managed to achieve the kind of sound that feels good to me (which was an evolution of what i had done with my SB Live! 5.1 before that) and i find it very hard to get something similar (let alone better) from other cards or MBs, etc. Even the SB Z, which is probably a better sound card than the Audigy falls a bit behind IMO in my ears.
I would describe the sound as "rich" and "powerful" with strong bass and very clear high frequencies. The sound i get from MBs (even after a lot of tweaking) don't have as crystal clear high frequencies, the sound isn't as "strong" and i always feel like it's a bit muffled, something i can't get rid off.
Now, while anecdotal, i have to also mention that a lot of my friends or other people that were interested in this agree with me, after they heard my setup. So it's not just me. But i do agree that there is no other way for someone to be able to tell unless they hear themselves and compare different setups. Reading words in a forum doesn't really cut it.
I'm glad to hear that you found your personal perfect audio setup (at least currently, who knows what the future holds for your audio endeavors) and am equally sorry for the problem you have that it seems like you have to get rid of said card. I understand the frustration of fighting an uphill battle just to keep your favorite hardware working.
I think this is a two-sided issue we have with how fast technology is advancing, where on the one hand we have insane feats just leapfrogging themselves like GPUs and CPUs since 1990s till now, or Sound chips and onboard audio since then, which even todays cheapest on board audio is worlds apart from the very first on board audio chips. But also because this advancement tech becomes obsolete and unsupported rapidly (Phones only getting 1-3 years security updates, devices being left unsupported once the next gen releases or in your case your sound card.)
Also, since you also said it yourself, it is anecdotal.
I don't imply that you are lying and very much believe you that your friends said this, but I think such subjective things are very hard to pinpoint to one component.
We're here debating over the stage sound gets generated in, but what if the difference actually happens in the headphones or any other part of the chain? Headphones also have different levels of sensitivity and can reproduce different frequencies better, which is why there are some better suited for bass-heavy songs, others for rock, mastering headphones try to be as neutral as possible but afaik to get really close to neutral one has to sell a kidney or more.
So maybe your setup does generate a hearable difference, but for someone with different headphones there would be none? My "evidence" is also only anecdotal. I very well could just have a "taste" in music that I could reproduce easily between the Modi and my onboard audio chip.
The only fact that I can present is, that as of today, external and internal sound cards aren't apart as big as they were in the 2010's and earlier. I won't deny that cards produce better signals if you measure them, but from a technical standpoint, both are already very close to each other on paper, so if there indeed are night and day difference, it lets me assume it's not just one component but a multitude that harmonizes and skews the results one way or the other.
To not escalated too much into theoretical subjects, I would like to end this with the fact that I think everything that plays with anything that we as humans perceive, be it smell, hearing, vision or anything else, is inherently subjective.
We can compare all the "technical" aspects like SNR, dB etc. all we want. Maybe the very best of the best DAC's out there would sound bad to you, and I mean that would be a fact for you. It wouldn't matter that every engineer on earth would disagree with you. It would still sound bad to you and at the end of the day, how it sounds to you is all that matters.