Objectively speaking, there is no reason why the Xonar Essence STX would not be perfectly fine to drive the T1. It is false information that it has "not enough power" to drive those headphones, and is mostly based on audiophile prejudice against sound cards, and the (incorrect) assumption that an expensive headphone requires an equally expensive amplifier. In fact, this card has worse - but usually still not bad - sound quality when driving low impedance headphones (and especially some IEMs), so you can ignore advice from those who suggest it is "not suitable for driving anything higher than 32 Ω". The only way in which high impedance is "difficult to drive" is that it needs more voltage for the same power, but the amplifier on the card can output enough voltage (up to about 7 Vrms without audible distortion) for almost everything, including the T1.
None of the FiiO amplifiers would "upgrade" the Xonar STX for the purpose of driving the T1. The T5p is basically a closed version of the T1, and sacrifices some sound quality for isolation. If you do not actually need isolation, it is a worse choice for the price, regardless of what you drive it with.
Note however, that the T1 might not be the headphone you want anyway (unless you have already listened to it, preferably compared it to others, and liked the sound). It is overpriced compared to the similar sounding, but 4x cheaper DT880, and many people do not like its treble. If you are not sure yet what headphones would be the best to buy, you may also consider some other models. The headphones are the most important factor determining the sound quality, so it is important to choose the best ones for your tastes.
