Typically, EQ can never be used directly on BT. Bluetooth is a transport, not a rendering protocol. It does nothing but move a digital stream of data from one place to another (just like WiFi/Zigbee/Ethernet). It has no idea what data it is moving; as far as it is concerned, a jpg is no different than a flac stream. Only the stream header has any info on how the endpoint should handle the incoming raw data.
The audio data is encoded directly to a digital stream in BT protocol, then sent to the receiving device. It is up to the end device to decode the data stream and render it into analog audio (the DAC). It can then have effects/eq/leveling applied. In order for a transmitter to apply EQ to *anything* going out BT, it must first decode the digital data, apply the effects, and reencode into BT - that would take a lot of processor power. Some devices have a hardware DSP chip that can do this internally, but for most SoC devices, it would be up to the SoC designer to integrate the functionality, which increases cost and power consumption. With most DAPs, the internal DAC is bypassed when sending out via BT, so no processing is handled by the DAP itself.
Wired headphones can EQ easily, as the device decodes the audio data in it's built-in DAC on it's way out to the headphones. Once the signal becomes analog beyond the DAC (or inside the DAC) EQ and effects can be applied to the analog audio stream before amplification. In the case of BT, this stream would exist not in the transmitting device, but at the endpoint (speaker, HP, or external DAC).
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