What you want is a "speaker level to line level converter." Google that phrase, WITH the quotes, and you'll find a number of choices.
Your Bose unit is not a preamp. The term "receiver," in the context of an audio system, is usually used to refer to a unit which combines a preamplifier, a power amplifier, and an fm or am/fm tuner in one unit. It sounds like that is what your Bose unit is.
What you are trying to do is to connect the output of the power amplifier section of the Bose to the input of the Bedini power amplifier. Separate power amplifiers, like the Bedini, are designed to accept "line level" signal inputs, which are at lower voltage levels, and higher impedance levels, than "speaker level" signals.
Speaker level signals are those which are capable of being directly applied to speakers, and are generated by power amplifiers, or by the power amplifier section of receivers, or by the power amplifier section of integrated amplifiers (which combine a preamp and a power amp on the same chassis, but do not have a built-in tuner).
Your Bose receiver is apparently capable of driving speakers directly, since it has speaker terminals, and since it is referred to as a "receiver." The reason for putting the Bedini in the path between the receiver and the speakers would presumably be to have the capability of providing more power to the speakers, and/or because the Bedini may be able to provide better sonics if the speakers have hard-to-drive impedance characteristics.
BTW, conventional speaker level to line level converters (that present a high impedance to the first amplifier's speaker terminals) should never be used with amplifiers that have tube output stages, or damage to the amplifier may result.
Hope that helps,