I have a Yamaha MT4X, in perfect working condition.
Here is what I recommend :
1) Use Chrome tapes (type 2). This unit doesn't work well with standard type 1 tapes, especially in the high frequency range. If you use type 1 tapes, make sure you don't overdrive the sound and keep it from hitting the red too much/too frequently in the display. Always record at double-speed.
2) You can order chrome tapes at
https://www.duplication.ca
3) Use DBX. Seriously. It'll reduce the noise floor a ton, everything sounds cleaner/better. Frankly I find the MT4X to be rather noisy without using DBX.
4) Keep it clean. Use a q-tip and alcohol for the head, dishwasher soap water (with q-tip) for the rubber parts. Use a demagnetizing cassette, if you can find one. I bought one on ebay, came from the UK, works well, worth it. Try cleaning the unit before changing the head, see it gets rid of the hiss you are experiencing.
4) Don't listen to people who tell you to give up on tape. Art is as much as process as the result.