Unfortunately, I think the answer to your question might be "both." As good as the phono stage in the CAT preamp is, the performance of the Manley Steelhead is significantly better. However, I suspect you will find that the line stage preamp in the CAT is better than the line stage preamp in the Steelhead.
The progression of components in my system was from the CAT preamp, to the Lamm L2 (line stage) and LP2 (phono stage), to the EMM Labs DCC2 (line stage and DAC) and Manley Steelhead. The Lamm phono stage significantly improved on the performance of the built-in CAT phono stage, and the Manley Steelhead edged out the Lamm phono stage by a less significant margin. I have occasionally used the preamp in the Steelhead to drive my CAT amps directly, and I have found that, in my set-up, the Steelhead line stage seems to be somewhat lacking in gain and dynamics. However, your experience might vary from mine, especially given that your speakers are more efficient.
It might be possible for you to buy the Steelhead and an older generation CAT preamp used for a combined amount that is within your $7,500 budget.
The progression of components in my system was from the CAT preamp, to the Lamm L2 (line stage) and LP2 (phono stage), to the EMM Labs DCC2 (line stage and DAC) and Manley Steelhead. The Lamm phono stage significantly improved on the performance of the built-in CAT phono stage, and the Manley Steelhead edged out the Lamm phono stage by a less significant margin. I have occasionally used the preamp in the Steelhead to drive my CAT amps directly, and I have found that, in my set-up, the Steelhead line stage seems to be somewhat lacking in gain and dynamics. However, your experience might vary from mine, especially given that your speakers are more efficient.
It might be possible for you to buy the Steelhead and an older generation CAT preamp used for a combined amount that is within your $7,500 budget.