I'm not going to answer your question about point of diminishing returns in regards to DACs. However, I will say this having used the STX as a DAC/amp myself in the past: your STX is not good enough, IMO.
The performance of a DAC is based on three main things: the power supply, the DAC chip, and the output stage.
While the DAC chip itself is important, don't obsess over it. A great DAC chip with a poor power supply will sound worse than a mediocre DAC chip with a good power supply. Pretty much any modern DAC chip will sound good if the power supply and output stage are well built, although some prefer older R2R DAC chips, such as the PCM1704. The problem with these older R2R DAC chips is that they are no longer manufactured (scarce), expensive (due to scarcity), and are usually limited to frequencies of 96kHz and below. The latter is only a disadvantage if you care about playing really high frequency music files and/or future-proofing.
Having said that, I think it's obvious where the STX falls short: the power supply. The STX gets its power from the computer's PSU, which itself can be fairly noisy, especially considering the change in power loads that occur due to changing computer conditions (loads on the CPU, hard drives, video cards, etc). The STX also exhibits problems from two other sources. First, it is nearby many other electrically-noisy computer components. The metal shield on the card may help some, but it won't shield it from everything. Secondly, there is no simple output stage (i.e. a line out connector) in the STX. Everything goes through the regular headphone amplifier stage. In other words, if you run the signal from the STX to another headphone amp, you are essentially double-amping the signal, which is a big no-no. It might sound OK in some circumstances, but you will probably run into problems doing this sooner or later. For example, when I tried running the STX's outputs to an integrated speaker amp, I would get tons of noise unless I turned down the STX's internal volume, which made the overall volume too low.
Many people have tried modding the STX (different opamp chips, external power supplies, etc), but I just don't think you will achieve the same results as a decent external DAC.
When I upgraded from the STX to my current DAC, a Channel Islands VDA-2, I noticed a big difference. The noise floor was lower, and I heard more details in the high and low frequencies, where the STX seemed to just roll-off. The mid frequencies sounded about the same. The sound stage also became a bit larger.
Am I satisfied with my current DAC? Yes, but I would like to upgrade sometime in the future to a higher-end DAC. I am currently stuck between purchasing one of the Audio-GD higher end DACs (Ref-7 or NFB-7) or building a Buffalo II DAC using the Twisted Pear Audio's kits.
Short version: I highly recommend you invest in a decent mid-range external DAC (around $500). You will notice a definite difference between that and your STX, IMO. Whether or not you decide to get a high end DAC after that is up to your ears.