The 159 should be able to take most any #6 nib that is on the market. I say *should* because I don't own one and sometimes feeds are a little individual, but I have seen a lot of people put other nibs in those pens. As to the x159, that nib is essentially a #8 nib, and the only after-market nib in that size you can buy is from Magna Carta (India0, a steel nib that may or may not be an improvement on the Jinhao nib. Yes, there are other #8 nibs but the makers don't sell them outside of the pens, so you'll have to look for 2nd-hand nibs.
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