Harvest Moon is the only user I know that has had his valve change outside of warranty.
For starters I would suggest you check to see if the tractor has the jerky hitch or not. Get a heavy implement hooked up, get the rpms up to 2k and lift the hitch slowly as you would if you were trying to make adjustments. It will be quite obvious to you whether your hitch and thus valve is acceptable or not.
If it is the mega jerky variety, it will move violently in 1 inch increments.
If the tractor is still under warranty, I would first try to get the dealer to get the valve changed to the one from the L3010 (old grand L). Harvest Moon has another thread going where he has tried several things himself to get the hitch smooth and so far he has found the valve replacement, as Kubota had done for us, as the best option. He's got part numbers in it and so on and should be able to tell you what the valve change cost him. I'd post you a link but I'm on my blackberry right now.
If you can't get Kubota to cover it, maybe make a deal with the seller as he sounds mechanically quite adept, so that you would cover the part and him the labour perhaps? Is he a kubota or other tractor dealer perhaps?
All in all the L3400 is a work horse for the price and the original jerky valve imho is the weakest thing on the machine. While I fully believe thay the Grand L offers a lot more value for very little more money, there is nothing I have thrown at my L3400 that it has not been able to do effortlessly.
I would not depress the clutch once the pto is engaged except to disengage it. I do the clutch depressing and turning pto off in one movement to minimize racheting so as to not wear the over running clutch that rachets.
Only other tip is to keep the Pto engagement cable lubed. Do it every time you grease othe components - every 10 hrs or so.
Good luck.