The codex in iphone is limited to AAC. AAC is a very processor-heavy codec since it uses psychoacoustic modeling to transmit data. Iphone typically weighs energy efficiency over performance and will encode AAC Bluetooth (BT) to a lower bit rate and quality. Therefore, the sound quality thru iphone bluetooth is not good, worse than the sound quality transmitted from laptop bluetooth. The bluetooth from laptop/PC is very close to CD quality (44.1Hz sample rate). So if you want to use laptop as a streamer, invest a quality DAC/BT with the money you have planned for a BT receiver. Loxjie D30 is a solid all-in-one DAC/BT/headphone device (new version $260) for your consideration.
If you still want to stick with iphone as the streamer, purchase a good external DAC and use the lighting to USB cable to connect iphone to DAC and then power amp. Amazon sells a lighting cable that has a Y spliter into both USB Type B (to DAC) and USB Type A (to charger) so you could charge your iphone while listening to music. Quite convenient.