I'd like this discussion to remain simple and about the topic. This is not about balance of stealth/detect as a whole, it's about the power of this single level 1 spell.
Claim: I don't think Amplify should be a straight +20 listen.
Reasoning: In active detect mode, you make a detect roll 5 times per second. The duration of amplify on scrolls/items is 18 seconds. That is 90 rolls. EDIT - Incorrect, sorry for the confusion. You roll listen once per round, but it checks for circumstantial modifiers 5 times per second. Why? Who knows. But I still stand by my claim that a flat +20 is a bit much, considering the availability of the spell.
There is no reason that a level 1 spell should give a +20 to a skill for 90 consecutive rolls. Especially a skill that you only need to pass a SINGLE roll on to detect someone. With this spell alone, no gear, no skill points, and no WIS mod, you can spot anyone who has under 40 move silently, just by using the listening horn that has no restrictions, 40 charges, and costs 1k. True Sight has been altered here to only give +1 per level, and I think that is very appropriate (That ends up being a +11 from scrolls, and a +13 from the player made potions). Other spells that are useful on items received a nerf too. Knock, for example, used to be a flat +10 open lock here. It was revised to be +1/2 levels because any non-caster could just pop a scroll or Knock item and get the full benefit as well without even being a caster.
Solution: Make Amplify scale with caster level. High level bards are very skilled with this kind of thing, and so they will be able to get the full benefit of the spell. It will also help to alleviate some of the unbalance between listen/move silently on the server. At the same time, there will still be detectors who can easily spot/listen people by simply putting points into the skill and using this as a small boost rather than an instant detect.
Rebuttal: A very common claim for this is that "Most people don't have a ton of consumables, and can't walk around with this spell on all the time". The cost for these scrolls in the shops that sell them is about 33gp each, if I remember right. I would also point out that this spell was never intended to be a spell you walk around with active all the time. However, if you want to avoid a sneak listening in on your conversation, all that is required is that you go into a room. Then, you shut the door and then use all your short term detect buffs. From there, they have nowhere to go and you are going to detect them with the 90 rolls that happen over the 18 second time period of this spell. Even if a room is not available, you can just pop this spell wherever you are (Other than the Outskirts or Barovian city maps, the entire server is basically free reign to do this with no concequences), and anyone who is in stealth nearby will have to beat all 90 of your rolls.
In closing, I think this would help balance out what many perceive to be an unbalanced mechanic. The fact that this stacks with Clairaudience/Clairvoyance (Other thread, should be addressed separately but still worth noting) only makes it even stronger. It would still provide a small boost to non-casters, and allow casters with levels in their skill to still have the full on power of the spell.
Alternative: At least reduce it to a flat +5 or +10 if you do not like the scale idea. 20 points to a skill is quite excessive when you consider the stealth's counterpart (One With the Land) only gives a +4 to Move Silently. Also, "Silence" items last only 18 seconds as well, and require you to exit stealth mode to use them, which completely negates the entire reason you want to sue them.