Carbonation will affect how you perceive bitterness. Ambers are generally carbed at around 2.5 volumes, which is typical for many American style light and medium color beers. This will make these beers more bitter; however, it will not be hop bitterness. Ambers are also more malty than pale ales and most IPA's. This means that you need more hop bitterness to balance the maltiness. You can continue to increase the IBU's higher than many Pale Ales until you get a bitterness that blends well with the biscuit and toasty character of an amber.
If your beer tastes more bitter than you like, even though you brewed to style (or from a kit), you might want to warm your beer to 50-55 degrees before drinking it. I find that the malt character really comes out at this temperature range.