1N4148 and 1N914

In many cases, these diodes are used purely to establish a diode drop, i.e. a .6V reference (often used in pairs, thus 1.2V or so) in order to bias a transistor. If that's the case (it's been a while since I looked at a Leach schematic), then either part is way overrated, so you can use either. I believe the 4148 is a 'fast' diode which may or may not matter, depending on where it is in the circuit and your personal philosophy on fast vs. slow diodes.

I was just looking at the Leach amp website for some other info and saw this:

"You should not use substitutes for the 1N4148 fast switching diodes. If the diodes which you purchase do not have 1N4148 printed on them, do not use them. I have seen amplifiers not work properly because the vendor supplied a substitute diode that was not a fast switching diode. The slower diodes cause the protection circuit to be activated on the zero crossings of the signal and this produces large amounts of distortion."

But both 1N914 and 1N4148 are both fast switching diodes (4ns). So try 1N914s (or get rid of the protection circuit).