Where are you located? You need to be aware most countries have limits on the maximum EIRP (power). In Europe most countries are limited to a maximum of 20dBi (100mW) @ 2.4GHz. Assuming your AP is using an 8db Omni antenna with your amplifier, ignoring cables losses, you would be emitting 38dBi, nearly 8W. Maximum EIRP in the US and some other countries is 36dBi, but only with directional antennae, I believe the maximum for Omnis is 30dBi (1W), this means your Wireless TX power must be 22dBi or less, approx 200mW.
What is the gain of the antenna at the AP and Client ends? What length and type of coax cables are you using?
Using the Link Calculator at http://www.mikrotik.com/test_link.php
, if you are using something like 19dBi gain antennae you definitely don't need the amplifier.
If the AP antenna is an Omni, say 8dBi gain, you should still be seeing signal powers of 61-66dB @ 6Km this should be more than enough power, depending on the receive sensitivity of your wireless cards it should provide a Link Margin of at least 10dB which should be more than enough to cater for most weather conditions.
If your close by Clients at 1Km and 2Km are pushing out too much power they will potentially swamp the longer distance clients. You need to understand Wireless power theory and use a link calculator as in the URL above to determine the power levels required for each client. Each client's power should be tuned to a level that is high enough to provide a stable connection, but not so high it's causing interference with other units.