In search for an AMP for my GSP500

If you don't need any of the features of the DLX (one touch presets) or the STX II (7.1 output), my suggestion is to stay with the Hel.

well a dedicated soundcard includes both an AMP (up to 600 ohms if I recall correctly) + works as a DAC, isnt it better than external devices such as the HEL?

as I've previously stated I used PHOEBUS SOLO + PC360 since late 2013 (Windows 7) up until late 2021 and the quality was amazing

The Hel is a DAC + headphone amp. The ASUS ad copy mentions 600 ohms to indicate it can drive high impedance headphones such as the Sennheiser HD 600 / 650 which are 300 ohms nominal.

The GSP 500 are 32 ohms nominal impedance, so they don't need the large voltage swings that an amp designed for 600 ohms impedances would output. Furthermore, they have a fairly high sensitivity at 107 dB / 1 V RMS so don't need too much power to produce deafening volume.

In any case, either a sound card or an external DAC + amp could get you better sound from your PC.

what's better in your opinion quality wise? powering the GSP500 with the HEL or Im better off buying the Asus's RAID DLX ?

Specs wise, I think they’re in the same ballpark except for output impedance where the ASUS is much higher. Based on that, it’s probably a better choice to go with the Hel as the frequency response won’t be affected very much by your headphone’s impedance curve.

You can find measurements for the Hel here and for the DLX here.