I've checked the issue, here is what i've found out:
If you set the sample volume (or leave from start) to 128 (+0db) you will get EXACTLY the half of its "strength" in instrument tab. Try to set the global sample volume in general tab to maximum 256 (+6.93db), you will get the exact same volume of the sound in sample payback as in instrument playback.
The solution of my research together with your report to clipping "if the sample peaks near 0 db", is that the sample tab playback most likely seems that allways uses the maximux extra (+6.93db) for playback, no matter what you set in "sample playback" volume in general tab, and the instrument of the sample seems to play it in the most proper way, and follows the settings.
Because of this (and many other reasons) i reccommend that you use instruments in your songs, rather than samples, but if thats not available to you for any reason, setting "Global Volume" in "Samples tab" to 32 can be a solution for you. That will get you the proper (+0db) playback without clipping on loud (near 0db peaks samples), narutally setting it to 0 will make it silent and leaving it on 64 will give the (+6.93db) boost (and will clip naturally on loud samples =).
Hope that it helps.