My AVR is at the back of the room. It's roughly a 25' run. To convert from the standard high-level outputs to pre-out on the Marantz SR 8015 would need a conversion from standard two-wire to RCA.
You don't convert from the preamp out RCA to speaker wire. The preouts on your AVR are designed to be connected to a power amp or another preamp with an HT by-pass input which is then connected to a power amp. The power amp has the speaker binding posts which are connected to the high level inputs of the sub amps and to the speaker binding posts. This can either be done by connecting two pairs of wires to the power amp's binding post and running one pair to the speakers and the other pair to the sub amps or run one pair from the power amp to either the sub amp or the speakers and daisy chaining a second wire to the part not connected to the power amp.
If you want to keep low frequencies from the amp powering the speakers you need an inline filter between the preamp and power amp. The filter needs two legs per channel. One leg has a cap that will limit the low frequencies going to the power amp and the other leg is a by-pass that sends a full range signal to the sub amp.
This is how I had my Marantz SR7008 (and now have an Arcam AV40 preamp/processor) connected. The preamp outputs of the Marantz (now Arcam) are connected to the HT by-pass input of my two channel preamp. This preamp is connected to an inline filter with by-pass legs. The filtered legs are connected to the monoblocks and the by-pass legs are connected to the RCA inputs of the sub amps.
The AV-1/RS surround speakers are connected to the AVR (AV/Pre-Pro) surround speaker terminals and the sealed servo sub is connected to the AVR (Pre-Pro) LFE/sub RCA pre-out connection.
When listening to two channel music, the two channel preamp is connected to whatever source I'm listening to and it controls the main volume. When watching a movie, the two channel preamp is set to the HT by-pass input. This allows the AVR/Pre-Pro to control the main volume.
For my type of setup to work, you need a two channel preamp with an HT by-pass setting or know at what volume position the two channel preamp is at unity gain. If your preamp does not have added gain (either a passive or active unity gain) the needed volume setting is full on/wide open. If your preamp does not have an HT by-pass input and has a gain stage, you need to know the volume setting where the preamp is at unity gain. For example, if your preamp has a volume control that goes from 0 to 100, and the company says the unity gain position is 76, set the volume at 76 when using that input. If your preamp has an HT by-pass input, that should automatically be set at unity gain.