I'm assuming you already have the Modi 2, if so, I can suggest you three amps
- Little Dot I+ (with NOS tubes and, why not, upgraded opamp), a budget hybrid amp. I own it, and it drives well anything from low impedance (T50RP mkII, HE4XX, M50x) to high impedance (HD6XX) and adds a bit of warmth to the sound (not as much as a DarkVoice 336SE, but you can hear the difference from a bugdet solid state DAC/amp).
- Little Dot mkII, the full-tube brother of the LD I+, basically the same but it also uses tubes for the amp section and not just the preamp. It'll add a bit more tubey-ness than the LD I+ but won't drive well low impedance headphones.
- DarkVoice 336SE. This might be a bit above your budget, but it's a excellent OTL tube amp for this price. I bought it through MD, unfortunately it decided to stop sounding from the right channel after a few months of usage, but I'm trying to contact the manufacturer and if that doesn't works I'm confident I can fix it with my basic electronics knowledge. However, bear in mind this kind of amps only work well with high impedance headphones, don't even try to plug multi-driver hybrid IEMs or planar headphones. In fact, the 150ohm your HD 58X has are not ideal, but it'll drive them quite well.
If you don't have the Modi 2 yet, I suggest you to:
- consider getting another DAC, Topping D30, it shows better measurements and costs about the same (even less from some eBay sellers or when available on MD).
- if you're 100% sure to get the Modi 2, you should consider going solid state and buying the Magni 3 along with it: from what I read it offers ridiculous value for the price, it's versatile and very powerful, you won't "need" another amp (like you would if you buy, for instance, a DarkVoice 336SE).
If you need any tube suggestions for the Little Dot or DarkVoice, just let me know. Good luck!